In my game, the player can acquire only 2 types of guns, a Sleep Gun(tranquilizer gun) and a charge gun(used for taking out robot guards). My problem is, I don't want the player to kill a human in my game. I could do it so that if the player has the charge gun equipped and attempts to use it against a human, it will just say "command not allowed" or I could allow it but punish the player, for example, diminishing the health of the player.

I know in Metal Gear Solid, if you hit/kill one of the main characters the game, your health will be diminished and it will end if she is killed by Snake. However, in the same game in one of the areas, they disable the use of weapons because in-story it makes sense, but, they could allowed the player to use weapons and just punish them(i.e ending the game or getting caught and having the escape the holding area).

Which is the better approach?

Charge gun? Like electricity? Is this a stealth game? You could have it be that the charge gun will work on humans sort of like a Taser. Except that the human will always cry out, alerting anyone nearby, and doesn't stun or knock them out for very long, so is worse in every way over the dart gun.

Why not just make the charge gun not do anything to humans?

Maybe instead arm your player with an electromagnetic pulse gun to use against the robots?

Charge gun like electricity, right?

Just make your guards wear rubber shoes so they can't be electrocuted or something.

Also make sure you can't shoot people multiple times with Tranquilizers, they may overdose.

Having environmental conditions to prevent certain usage of guns would work better. But then you'd be restricted to only allow other human players in that area of the game.

It really depends on what type of game you're creating.

Perhaps give us a little more info?

punishing that does not make sense in the context of the game world is an immersion breaker and bad design.

if you shoot a goodguy, cops chasing you make sense. you health going down does not. that's a "bad designer! no twinkie!" if i ever saw one.

you should try to make something not possible due to circumstances that seem wholely reasonable and logical in the context of the game world. so you don't break immersion.

so what you want is an EMP gun, which does nothing to humans, except perhaps with repeated exposure at max power for decades on end. (the whole EM fields and cancer thing - IE don't build your house under high voltage power lines).

Charge gun? Like electricity? Is this a stealth game? You could have it be that the charge gun will work on humans sort of like a Taser. Except that the human will always cry out, alerting anyone nearby, and doesn't stun or knock them out for very long, so is worse in every way over the dart gun.

It's a spy text adventure game. So yes, stealth game if you want to call it that.

In many games EMP weapons can be seen. This is a kind of weapon that creates electro-magnetic pulses that disable vehicles but ignores creatures.

Here, I have another idea: computer viruses. Just replace your "charge gun" with a virus-deploying mobile device or a hacking-pad, and everything will turn out fine(and a little nerdy :) ).

Oh I have some more ideas that may be useful for you:

-Human guards wear special reflective armors, firing charge weapons will reflect the pulse into player himself.

-The guards are previously the player's friends or whatever. Hurting them will cause mental pain, and maybe will disrupt the user's control for a sec or two.

-The player believes in some religion or is cursed. Hurting others will hurt himself through some mysterious way.