For some reason.... you rarely see the ability to kill an NPC in a game (I've yet to experience even one).

Now, NPC is a rather vague term that seems to include teammates in team-based battle games (like Halo coop mode)

For the sake of this, I am limiting NPC to only the characters in an rpg game that give quests or exchange helpful dialogue. And a bunch of games have ways to deal with players trying to attack an NPC.

Dragon Ball Z GBA 2 - The A for melee attack is also the "talk to them" button. Energy blasts pass right through NPCs.
Runescape - There is simply no option.
Pokemon: You can only call out pokemon in battle mode where there are no NPCs to be found.
Sonic DX: The NPCs are just walls that talk. Spins go around them, homing attack doesn't target them.

Now part of this thought came from a video about "If Pokemon Were Real" where there is a weak electric pokemon fighting a strong cocoon, and the protagonist realizes, why don't I attack the actual enemy instead of his pokemon?"

Is there a reason many games are so adamant about not being able to kill the NPCs (excluding killing them as part of the story as I hear how it is on God of War. I mean to deviate from the obvious plot by killing the ones who are supposed to give you the quests) Are most devs too lazy to accommodate for story impacts, is a plotline of that degree of nonlinearity just not realistic, or what?

The closest that any game has come to killing NPCs as far as I know is Pokemon Mystery Dungeon Explorers of Time/Darkness where you can try to kill a shopkeeper, but the shopkeeper turns out to be almost impossible to kill (if you can kill him, you may take all items from the shop). But outside of dungeons, you can't attack anyone. Are there any better examples, or is killing NPCs just taboo?

I'm not a big rpg player but in the ones I played recently (Skyrim, Amalur) I could kill almost whoever I wanted. Exceptions being a few that are either instrumental to the plot or children.

IIRC EverQuest you could kill any NPC. And like Japro mentioned, in the ElderScrolls series you can kill any (or at least almost any) NPC also (Never played Skyrim, and only played Oblivion for about 10 minutes, but Morrowind I could kill who I wanted).

Both those games (Morrowind and Everquest) are almost ten years old or older. I'm not up on recent console games or MMOs, but if I can name two ten-year old games that have it, there are probably dozens of games since then that also allow you to.

Ultima Online you could kill npcs, they eventually made some invulnerable, but early on you cold kill every npc, later on you could kill a lot of npcs.

I made over 1 million gold (300-400 gold per kill) with a bard enticing (it was a skill to lure creatures with music) npcs out of guard zone, killing them, taking their gold, clothes, shoes, and sword and selling it to other npcs.

Thanks for examples!

However, I understand that most of these (besides Skyrim) are mmorpgs.

So how does each deal with the killing of an NPC who would give a plot-important quest? Or is the main quest given right from the start?

Suppose NPC Bob were going to tell me that to kill the Spider Monster I first need to go and gather Red Fruit from the Underground City of Nephilim and I killed Bob before he told me this. Would Bob respawn for me to get a second chance at getting the information, or would I have to travel until I find someone else who also knows how to kill the Spider Monster? Or would the tooltip tell me I need this red fruit while I am trying to kill the spider without it?

More generally, what happens in those games when you kill an NPC who would have otherwise given you crucial information towards achieving an important goal?

I Skyrim the "main quest givers" can't be killed. Side quest guys are mostly killable and you can fail quest or make them inaccessible by killing random people.

In The Elder Scrolls 3 Morrowind and all Elder Scrolls before you could kill Every single RPG even if it destroyed the main quest and you couldnt finish the game anymore. I think it was best like that.

So how does each deal with the killing of an NPC who would give a plot-important quest? Or is the main quest given right from the start?
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More generally, what happens in those games when you kill an NPC who would have otherwise given you crucial information towards achieving an important goal?

In The Elder Scrolls 3 Morrowind and all Elder Scrolls before you could kill Every single RPG even if it destroyed the main quest and you couldnt finish the game anymore. I think it was best like that.

But it's important to note that when you did kill an important main-quest required NPC it lets you know so you don't accidentally save when you weren't wanting to.

When you kill one, a simple dialog box pops up saying:

"

[color=#2f4f4f][font=verdana,geneva,sans-serif]With this character's death, the thread of prophecy is severed.[/font]

[color=#2f4f4f][font=verdana,geneva,sans-serif]Restore a saved game to restore the weave of fate,[/font]

[color=#2f4f4f][font=verdana,geneva,sans-serif]or persist in the doomed world you have created.[/font]"

If you killed minor-quest giving NPCs, I don't recall anything warning you. It just keeps you from progressing along that quest. Oh well!

If Alice has a good item she'll give you if you clear the rats from her cellar, but you kill her, you can no longer complete that quest. However, you might just be able to loot the quest reward off her corpse. I don't know if that's common or not, but I think at least some of the time it was possible.

Another game were you could kill any NPC was my favorite game, "King's Field" for the Playstation 1 (super outdated game - a first person action RPG). There were no "important" NPCs, as NPCs only give you plot details, and nothing depends on their survival. Unfortunately, they don't fight back, so there's not really any reward or challenge if you kill the NPCs.

However, I understand that most of these (besides Skyrim) are mmorpgs.

No, "most of these" are RPGs. Another one not mentioned would be Fallout 3 (don't know about prequels).

So how does each deal with the killing of an NPC who would give a plot-important quest? Or is the main quest given right from the start?[/quote]
In Oblivion you simply couldn't hit the important NPCs. There's actually a variety of ways to make it seem like you could kill every NPC and still make the plot advance. For example through events and cut-scenes, or you can make it so that at the next loot a certain scroll/book is looted that advances the plot.

So it is probably not a good idea to let a player kill an "essential" NPC without informing them that they just reduced their chance of winning to 0.

Now, one last question --

From a player's perspective (as opposed to a developer's), is there anything that can be gained from an unkillable NPC? Or should all NPCs be attackable? Of course, some, like your mentor at the beginning of the game, will be too strong to kill.

The only reason I can think of to disable attacking NPCs is to prevent accidental attacks, which is why some games have an option to turn Friendly Fire off. But anything else?