Just to add to Polama's response, the player shouldn't be able to kill important quest giver by mistake as it can and will lead to frustration. Imagine the NPC gives you quest to kill someone and your target runs just past your quest giver. You miss the target and there isn't anyone who could reward you later for that quest. But if you add a challenge to killing quest giver then it may be interesting.

Regarding your title question - ability to kill important NPC in not really freedom, it's game mechanic. Whenever you choose to implement it or not you should provide "things to do" for player. Be it quests, achievements, or just hints of what (s)he can do now or where to go. If you leave players with "what now?" question too early or too often they may get bored quickly.

You mentioned GTA, so let's get it as example. There is always at least one quest on your map where you can go. You may do hundreds of other things, but if you run out of ideas this the hint of what to do next. The same goes for Minecraft - basically game build around freedom that encourage you to do your stuff. But there are achievements that guide you in case you are not sure what to do.

well for the by mistake thing there will be somewhat of a warning "This game allows you to kill everybody so be careful with who you shoot at" and there could be some indication who are important and whose not there needs to be work rounds like giving those NPCs bodygaurds so it actually becomes a challenge and you choose whither you want to go through the challenge but you can do it if you have the right equipment actually this is why I am asking to know what would happen the pros and cons

I am saying freedom because of other games that restrict the player and change the rules when they feel like it

as stated in other replies, "too much freedom" is a matter of opinion, not fact.

my opinion: i like maximum freedom. but it should not destroy game play if possible. instead of not being able to get a quest because an NPC has died, or not being able to kill an NPC because they're required for a quest (skyrim). you should let them kill the npc, who over time gets replaced with a new npc who also has a quest (same , similar, or different).

this method doesn't break quest gameplay by killing off all quest givers over time - they RESPAWN!

it also doesn't resort to hokey BS like "set to essential" characters in skyrim.

deflinek has a good point about having to somehow deal with "friendly fire" accidents where the player accidentally kills an "essential" npc.

also the part about "things to do".

GOALS are the key. as long as the player has a goal they want to achieve - quest related or otherwise - they will remain motivated and engaged.

examples:

playing a pacifist monk in skyrim (no weapons, no armor, no spells, no scrolls). quest are mostly about killing things. thats lydia's job. mostly to get gear to improve for smithing experience points. so goals become things like getting 100 smithing and enchanting, and disenchanting one of every type of magic item. needless to say, since a lot of the action in skyrim is about combat, this can get a bit dull. also, there's no hand-to-hand skill in skyrim due to mass market streamlining. so i'm still working on the "rules" for playing monks. i've just added unarmed hand to hand, and am contemplating daggers, poison, bows, and sneaking - the non-pacifist version of a monk, IE a ninja. and that right there gives me enough goals to get yet a few more evenings of gameplay from skyrim, which i've played to death 100 times over already.

trying the "slave society" strategy in rome2. attack to capture slaves. slave based economy. slave related buildings. i came up with a dominant strategy for total war titles long ago (hint: its really a game about public order management - just like running a real world ancient empire probably was, and if you play the strategic game right, you never have to manually fight a single battle, the odds are always so in your favor), but this gives me a new goal in rome2.

the problem is when the player runs out of goals they CARE about. you may have many "things to do", but if they don't give a s--t about that kinds stuff - well... then you're screwed. they stop playing.

example: skyrim hard coded quests. eventually, you buy all the horse and houses. for every character you play. and you just don't care about getting red eagle's sword for the fourth time with this character, and the 67th time since you first started playing skyrim. that's when you've lost the player.

that's an idea re-spawning but that would take away from the system in other words the game is all about freedom and challenge but also about how freedom can create interesting results and how you have to deal with that freedom if you killed an NPC you have to live with that it makes it a tough desicsion that YOU have to make YOU have to live with but if killing them and after a while they respawn that just breaks an imersion and it becomes easier to kill everybody instead of adding this burden on the player

I thought the inability to kill 'key' enemy NPCs detracted quite a bit from Skyrim. Really broke my immersion that the game couldn't adapt to the player killing people that the player would obviously want to kill in the game's story.

I'm not really interested in games with story that let you kill any character. A well written story would rely on having certain characters around, and just can't adapt to every combination of characters being killed. So what you end up with is a story that has an important piece missing. If quests are lost because of it, that's even worse.

Sometimes when people talk about a game including this kind of freedom, it seems like they're more interested in making an experimental sandbox than a fun game or engaging story. It's like Warren Spector of Deus Ex explaining that he doesn't think it's worthwhile to talk about Fun in games, and instead he makes a game where "there's no one to talk to you" and "no one to help you", because you killed too many people.

I think being able to kill any character probably works better when there's less story, so they're not that much of a character to begin with. You can make a world adapt to this in a procedural way without having to write dialogue for all the possible character death combinations. But make sure the player isn't unwittingly removing fun options because they killed some quest giver.

that's an idea re-spawning but that would take away from the system in other words the game is all about freedom and challenge but also about how freedom can create interesting results and how you have to deal with that freedom if you killed an NPC you have to live with that it makes it a tough desicsion that YOU have to make YOU have to live with but if killing them and after a while they respawn that just breaks an imersion and it becomes easier to kill everybody instead of adding this burden on the player

I don't think Norman Barrows was saying that you should respawn the specific character that died but have someone take his place. It'd make sense that if you were to kill one crime boss that one of his lieutenants would take his place? Or alternatively some other individual that sees a power vacuum and wants to take advantage of the opportunity. You still have to live with the decision. It might be that the replacement is going to really really want to come after you. Or perhaps he simply has some other new job that center on his own interests to help solidify his new position.

If this still isn't what you're thinking of then what exactly are the "consequences" that you're wanting the player to live with? If you don't have something else in mind it just seems as though ultimately the player is just decreasing the number of missions he's going to have available after he's killed everybody.

well what I mean with to much freedom is the game rules that exist rules of the game don't change for specific NPCs they all have to follow the rules in other words no NPCs are beings that can't be killed until I want to be killed in other words if the player hated the an NPC for whatever reason and didn't like doing missions for them they can outright kill them there are concequences for one the NPC is now not in the game there missions are now gone you can't do them but it was your choice what encourged me to do that more are games like Dark Souls where the rules are set and they apply to everything
also this video from Tom Francis the creator of the game Gunpoint

about that but I myself hate AAA games where you Have NPCs that aren't killable for the sake of the story and what
Norman Barrows
said about skyrim now think about this one NPC asks you to kill another important NPC and wanted to give you a big reward and now it's your choice take this cool reward and kill this NPC with his own storyline that you might like or ignore this mission and keep this NPC something like that

okay so basically way to much is just all objects in the game follow the same rules without any specifications and scripted things you get my point

Freedom in a game is not inherently bad, but freedom helps you determine where your game design actually stands.

A strict narrative experience provides you with little freedom and let's you uncover its world in a carefully crafted way (such as many story-driven RPGs: Baldur's Gate comes to mind).

A 'free' world is more sandboxed and leaves the player in charge of the order and actual nature of events that will happen.

These cater to different crowds, and not all players end up enjoying either of these options.

Some very creative minds are naturally drawn to the freedom of a sandbox world (self-expression is more important than the flow) while others will fail to generate interesting content/events within the environment.

Likewise, some people will greatly enjoy a well-crafted storyline while others will feel imprisoned.

What is YOUR target audience?

If you kill the NPC whether they are mission-critical or mission-initiating, then what's the consequence? What does your decision tree look like? Is there a way to fill the void if you go kill every non-minor NPC?

IMO, "too much freedom" only applies when you break the game or the immersion of the game. Otherwise, sky's the limit.

Let them screw up, but if they screw up big then end the game instead of letting them wander aimlessly. Like if they ruin the main quest, let the 'bad ending' cutscene roll, maybe make a note saying "Guess you shouldn't have killed Louis, eh?"

I thought the inability to kill 'key' enemy NPCs detracted quite a bit from Skyrim. Really broke my immersion that the game couldn't adapt to the player killing people that the player would obviously want to kill in the game's story.

I know what you mean and that's why I like the concept of killing quest givers if you feel like it :P

I'm not really interested in games with story that let you kill any character. A well written story would rely on having certain characters around, and just can't adapt to every combination of characters being killed. So what you end up with is a story that has an important piece missing. If quests are lost because of it, that's even worse.

Sometimes when people talk about a game including this kind of freedom, it seems like they're more interested in making an experimental sandbox than a fun game or engaging story. It's like Warren Spector of Deus Ex explaining that he doesn't think it's worthwhile to talk about Fun in games, and instead he makes a game where "there's no one to talk to you" and "no one to help you", because you killed too many people.

I think being able to kill any character probably works better when there's less story, so they're not that much of a character to begin with. You can make a world adapt to this in a procedural way without having to write dialogue for all the possible character death combinations. But make sure the player isn't unwittingly removing fun options because they killed some quest giver.

well for this game story will be more just about characters and each character's story as in it's just characters the main story isn't something that I am really focusing on there isn't something I am trying to say just a rise to power type of story without any message to tell but I do want to create interesting & cool characters or at least try to and make them hate other characters and you get to choose which characters you like and which you don't I guess I should have said

for the story there isn't a major interesting storyline it's just a bunch of mafia families fighting each other trying to get all the power and you come and choose who to side with or maybe you want chaos and you play each family on each other or you don't care about anybody and want to gain all the power for yourself so I hope this makes sense at least it does in my head :P