Just because you can...
If there's anything I've learned from playing games where I destroy entire armies with my awesome fighting skills, cause frame-rate issues with the sheer strength of my magic, or hold the fate of planets in my hands, it's that having the power to do whatever the hell you want feels really damn good. But as with all things, there have to be some limits, especially when it comes to things that are plot-relevant. Even if you're going to explode if your traveling companion Hey Listens you one more GODDAMN TIME, it makes sense that you aren't allowed to blow them off the map. I think we can all agree that that would be a bad thing, because if they're plot relevant, where would you be without them?
...Or maybe we can't all agree, because some games go out of their way to let you do just that. They're the definite minority in a world of frozen trigger fingers and bulletproof children, but some games really do give you the power to off even their most story-critical characters. Interested? I'll ignore the fact that your interest deeply worries me, and give you a rundown.
Look out! Spoilers for:
Heavy Rain lets you kill the heroes and let the bad guy get away
Heavy Rain might as well have been called You Fucked Up: The Game! because that's basically the entire experience. Playing as one of four critical characters as you try to figure out the clues necessary to save young Shaun Mars from a Saw-esque drowning death, you can mess up in a shocking amount of ways with terrible consequences. Every character has to dodge death multiple times to make it to the ending, and that's not even counting Shaun, whose fate rests in your hands. A fate that you can completely screw up. SHAAAAUN!
Then again, if you're a sick bastard into power trips, you might get a kick out of the fact that you hold the fate of every character in your already controller-filled hands. Though it might be hard to see going in blind, with either a first playthrough under your belt or a guide at your side, you can hand-select who makes it through to the end. Will Ethan, Madison and Shaun start a new happy family, or does everyone die, leaving the killer to escape unscathed? You decide!
Dragon Age lets you gut your companions
As in real life, in Dragon Age games it's generally agreed that you dun good if none of your companions get killed over the course of your journey. That's not to say they all have to survive though, and you can solidify yourself as a complete dick by killing almost anyone you want. It can be indirect, like leaving the trapped Qunari prisoner Sten to be executed, or in your face, like deciding who flees and who stays behind to get eaten by a demon. Or you could just piss someone off enough - making the wrong choice in the wrong circumstances with the wrong person standing next to you means only one of you is walking out of there (you, in case you were curious).
It takes a little more work to get to the killing floor here than in some of the other games on this list, since [Stab him in the face] rarely comes up on the action wheel. It can be done with some concentrated effort and a lot of checking dialogue tree results... or you could just not try to kill your best friends and allies. That's an option too.
Mass Effect makes you choose
If Dragon Age gives you some hard choices to make alongside the opportunity to be a complete bastard, Mass Effect turns both of those elements up to 11 and sends them into hyperdrive. It's not only possible for you to choose who among the Normandy crew lives and dies, but mandatory, since the series delights in pushing two characters in front of you and asking which one's going to be left to the mercy of an interstellar nuclear blast. Even when the storyline isn't bordering on sadistic, Shepard's choices throughout the game can result in desperately unpleasant consequences for his/her squadmates depending on how they're handled. Yeah, try to talk tough at Wrex about the genophage, or tell Tali about that awful thing you did to the Quarians. That should go over well.
Also, if that doesn't fill your power-trip meter, both Mass Effect 2 and 3 end with scenarios where any squadmate tagging along with Shepard can be killed if certain conditions are met. Love Liara, but regret nabbing Jack? Hey Liara, let's go do your loyalty errand before the suicide mission. I'm sure we'll get to Jack's eventually...
Dishonored lets you make things worse
This one's a bit deceptive, since early on in the game's events you'll get a big fat GAME OVER whenever a wayward arrow finds its way into an ally's spine. Damn game, lighten up! And lighten up it does as you start to massage the storyline to fit how you want things to go, which includes who keeps their pulse.
Early on you get to decide the fates of your adversaries, from mook to main. Will you kill the assholes who assassinated the Empress and put you in prison, or just horribly disfigure/hand them over to their creepily obsessed stalkers? Once you start making those ethically dubious choices, it's just a hop, skip and a blink from turning your judgment on your allies. The masterminds, the scientists, even the nice tutor who never did you any harm are all at your mercy in the game's final act. That can mean the difference between happily ever after for all, and a pile of corpses that's only somewhat figurative.
Dark Souls 2 lets you screw yourself over
Did you know you can kill the Emerald Herald in Dark Souls 2? Yeah, the nice lady who gives you the Estus Flask at the beginning of the game and wants to help you on your journey to break the undead curse? Like most of the NPCs in Dark Souls 2, she's not indestructible and you can totally kill her. Not that you would want to though, since she... wait, you already did it? Oh. Well, have fun never leveling up again, since you have to visit her in order to do that.
Yep, that's right: Dark Souls 2 gives you so much freedom in terms of who you decide to kill that you can break your game in the process. While virtually any NPC can fall under your mighty blade, there's little reward once the sick rush wears off. For example, you can kill the three old ladies in the hut at the very beginning (I mean, if you're an asshole), which eliminates your ability to Respec later. And even regular old NPCs will be hostile toward you if you attack or kill them for no good reason, even if you revive them later, so think very carefully before you unsheath. Seriously, this is Dark Souls. Do you need any more enemies?
BioShock 2 lets you kill a granny
While the first BioShock gave you the opportunity to either kill a bunch of abused children or not do that what the fuck, the little girls you choose whether or not to murder don't have their own unique identities. BioShock 2 chose to take things a step further: not only are the Little Sisters' lives on the table, but so are the lives of three plot relevant folks who end up on the wrong side of your Big Daddy powerdrill. And it gives you progressively better reasons to want them dead.
The game eases you into the concept of making Tough Moral Choices with Grace Holloway, a defenseless grandmother-type whose most egregious crime is being snippy when you break into her house and steal her stuff. Things get a little trickier with Stanley Poole, a guy who screwed over the protagonist pretty hard in the past, and Gilbert Alexander, a mutated whale monster who says he wants to live but left behind a recording begging someone to kill him when he was still sane. Well that's... actually kind of a tricky one, put that way.
GTA 5 lets you blow 'em all away
You probably wouldn't think there's much room in GTA for moral dilemmas, between running over all those pedestrians and beating people down for fun, but GTA 5 manages to pull it off by telling you to off your friends. After a heist gone very, very well catches the attention of the wrong folks, Franklin's given a tough choice to make: kill his psychotic but loyal accomplice Trevor or his criminal mentor Michael. It's a legitimately tough call to make, choosing between two friends who truly trust and respect you. Oooor you can say screw that, load up and go full Dirty Quarter-Dozen on the guys who thought they could play you.
Admittedly, there's no option here to not kill anyone - this is GTA, what're you thinking? But the choice is totally yours to either off your allies, or go the hard road and pick off your mutual adversaries one by one. While Option C is undoubtedly the friendliest choice, if you've been dying to stick it to one of those amiable jerks since you started this bullet-hole-riddled destruct-o-thon? Now would be a good time to let them know.
With great power comes great responsibility
While few of these games actively encourage you to brutalize those who trust you most, they make a point to give you that freedom and let you suffer the consequences. Don't expect to get back on the Emerald Herald's good side, is what I'm saying. Did I miss any ally-abusing extravaganzas? When you're given the opportunity, are you a great friend or the biggest bastard ever? Tell me in the comments below, my dear friends.