Hitman Blood Money allows many, many ways to kill people. A particularly satisfying example from the mission "A New Life" involves using lighter fluid on a grill and waiting for the wife of your target to start it and go up like a Roman candle. And by doing so, you orphan 2 kids since you need to kill their dad in that mission, all this happening on the younger one's birthday.
Rigged pyrotechnic display + Tank filled with oh-so-cooling water. And a shark.
If a bullet hits someone in the stomach but doesn't prove immediately fatal, the victim crumples to the ground and rocks back and forth before succumbing to the wound. Highlighting the brutality of the kill, the victim spends the next few seconds howling and moaning in pain.
There is no way around doing some level of this to Lenny Dexter in Hitman Absolution, from tomahawking him to smashing his head open with a bong to marching him over to a carriage of explosive crates and blowing him skyward.
Even if you decide to leave without killing him, because there are no nearby cell towers and the fact that you dispatched all of his friends prior, chances are he will die painfully from starvation, thirst, heat stroke, or buzzard attack. Honestly, the most humane way to kill him is a nonsilenced pistol to the head.
Metal Gear Solid 3: Snake Eater requires the player to hunt and kill animals for food in order to survive. However, it's also possible to use live captured creatures as weapons; for example, throwing live venomous snakes at people — a combination of both cruelty to humans and animals which is often lethal to one of them.
Metal Gear 2: Solid Snake had something similar: Apparently, Snake could actually kill the NPC war-orphaned children if the player decides this. Note that these weren't even Enemy NPCs, but just defenseless children whose only action is to talk. The only punishment is losing health, which is pretty lenient compared to being docked of a rank in the the first Metal Gear, having it grant you a severely low rank in future games (and in the case of Metal Gear Solid: Peace Walker, lowering your clearance rank after completing a mission), and also making some battles even more difficult than usual (case in point, The Sorrow battle in Metal Gear Solid 3).
You get a lot of chances to fuck with the guards in Metal Gear Solid 3. For instance, first, kill a poison dart frog and collect its meat. After that, blow up one of the guards' food caches. When they start complaining of hunger, toss them the meat of the poison dart frog you killed and watch what happens...
Even before that, in MGS2, they introduced disablement of enemy limbs, as well as radios. Shoot a guard's radio to short it out, then run up and hold him up. Proceed to shoot any combination of his arms and legs, and he'll stand there quaking in fear, the affected limb(s) bloody and limp. You can cripple up to three limbs; all four finally kills the victim.
Overt guard and scientist abuse is possible when dressed in the Major Raikov disguise. Victims can be physically attacked and fail to fight back, instead cowering before and apologising to their "commanding officer."
This is lampshaded before you get a chance to do it by a radio conversation with EVA where, to Snake's disbelief, she tells you that you can punch anyone in the face because Raikov is "just that kind of guy".
Raikov's entire role in the game is to facilitate this. You are asked to knock him out and take him to a specific locker to steal his clothes, but he has all sorts of hilarious responses programmed into him for players who fancied a bit of violence against a Raiden-lookalike. For instance, his borderline-Cloud Cuckoo Lander personality means that he is always ravenous and will eat anything you feed him, which occasionally causes stomach trouble, making him run to the bathroom (and if you knock on the door a few times he'll let you in). You can drag him around in his underwear and whether you kill him or just knock him out has no effect on the plot at all.
Deus Ex: Human Revolution allows players a multitude of choices to kill beyond whatever weapons you have in your inventory. From throwing Vending Machines and dead bodies at guards, to hacking security systems to have attack robots and turrets kill their own men (while watching it happen in real time on camera). Then there are the lethal takedowns, most of which involve impaling enemy and npc with large chunks of metal. The game doesn't often discriminate who you can kill, from hookers on the streets of Detroit to potentially your own boss near the game's end.
Some of the counter-attacks and combo kills in Assassin's Creed look and sound painful. One attack involves kicking out a guard's leg, and then stabbing the sword down through his hip and out his crotch...
Other such attacks include a counter kill option to break a guard's legs with an audible crack. A short sword combo kill in which Altair grabs his victim in a sleeper hold before breaking their neck, again with an audible crack. And who can forget throwing your opponents into the merchant stands, causing the stands to collapse and killing both your opponent and the merchant. Bonus points if you catch civilians in the collapsing merchant stand.
For the above leg breaking and neck snapping, one combo kill combines both: Altair kicks a guard in the knee hard enough for it to bend backwards and break, to which the guard lets out a quick scream of horror and agony. Once the guard is bent over, Altair finishes him off by getting the man in a headlock and snapping his neck.
Which is still nothing compared to finishing someone off with a normal attack and seeing them writhing in pain for twenty seconds before they finally die.
Not to mention some of the "high profile" Assassination animations, one of which involves you walking up to a man from the front and stabbing him in the eye in broad daylight with your hidden blade.
Considering how annoying some of the citizens are, especially the beggars, lepers, and drunks, it's not surprising when one feels the urge to start stabbing and slashing.
Not only that, but citizens simply love getting in your way when you're trying to stab a guard.
Sometimes, if the guards have enough room and after you've killed enough of them, they'll panic and flee. You have two choices — chase after them (since you're faster) and kill 'em with the hidden blade, or simply toss some throwing knives in their direction...
The approach to Jerusalem. A long, narrow path packed with at least a hundred civilians walking along, and you're on a horse. It's like the developers want you to maim them under your mighty steed's hooves...
Civilians will absolutely freak out if they see someone die suddenly. Want to see some chaos? Stand on the rooftops and fling throwing knives at the guards below, or perhaps sneak up behind an archer and shove him off his roof.
There is absolutely no penalty for killing guards at random. Stab one guy, wait for his friends to show up and investigate, stab them, rinse and repeat. It's quite easy to get caught up in this and carpet a street with bodies.
Assassin's Creed II introduces the ability to poison a person, causing them to go insane for a short while before dying. If said person is a guard holding a weapon, there will be blood. Bonus cruelty points for throwing money on the ground to attract more people. It's possible to get 30+ casualties this way, and because the only person you kill directly is one guard, there is no penalty.
You can also poison a civilian near guards and they'll distract the guards. While they're focused, assassinate them all without being noticed.
Get a bunch of guards chasing you and jump on a tight-rope run while they're on the roof. If they're close enough together, they'll jam into each other and fall to their deaths. Also works with thieves.
You can pick up bodies in this game, which adds all-new dimensions, like the following: Kill an archer on a rooftop, then grab his body and chuck it into the street near guards. The guards will walk over to investigate; use the opportunity for an aerial dual-assassination.
Some of the combo kills you can perform with the sledgehammer are hilarious in their brutality. Block of iron swung straight at a guard's crotch? Check. Hitting that same doubled-over guard again in the back of the head to lay him out on the floor? Check.
One of the barefisted counters lets Ezio break the victim's Adam's Apple.
One assassination technique you gain is that of hanging off the side of a building, then stabbing the target in the gut as they walk past before pulling them off the roof. If you find an especially tall tower with guards, you can use this technique, then, as gut wounds don't kill instantly, watch them plummet to their bloody death with the knowledge that they are still conscious. Bonus points if they hit a slanted roof and then the ragdoll physics sends them spinning halfway down the street.
Or you can just throw them off the roof and hear them scream. Then hear the civilians scream as they realize that someone just dropped out of the sky and died in front of them. It is not unknown to find someone still running in fear of the cruel deed you committed several minutes later. Congrats, they're probably scarred for life.
Using Ezio to quietly nudge a fishing civilian into water is a great way to kill as many people as you want without penalty or incurring the wrath of the guards. Also, the civilians never, ever resurface.
If you thought Ezio's regular unarmed attacks were brutal, you should see the world of hurt he can unleash after stunning a target with dirt.
Stabbing a guard through the bottom of the head up, followed by point-blank headshot.
Flooring a guard with a Groin Attack, then stomping on his head.
Throwing a spear hard enough to impale a guard and send him falling back.
Ezio's Counter Kill animations have also become severely more... violent than they were in AC2. The strongest knife, the Dagger of Brutus, even has its own kill-animations, including, but not limited to, Ezio trying to force a 5cm wide dagger into a person's eyesocket, only to turn the dagger 180 degrees, while it's still implemented in the victim's eyesocket.
Execution Rows. Ezio is able to kill a guard and then seamlessly move on to killing the next. And the next. And the next. All in one hit each.
If the player is feeling lazy, he can even call for apprentices to kill the guards for him.
Remember those Borgia couriers from the last game? Remember how you couldn't kill them without sending your notoriety through the roof? The same conditions still apply in this game. But apparently, they don't mind you having your apprentices do the killing for you.
The newly introduced lifts, which send you rocketing to the rooftops, do so by employing nets full of masonry as counterweights. If anyone happens to be nearby when you trigger one, well... The really fun part is getting a squad of guards to chase you, running to a lift, and waiting 'till the last second to set it off.
In Thief: The Dark Project and Thief: The Metal Age, you can potentially kill every single last person on almost any level. Granted, you can't be playing on the higher difficulty settings (as those make killing innocents, or on the highest difficulty setting, killing anyone, a "mission fail" condition. Also, some levels are pure stealth missions and auto-fail if you kill anyone on any difficulty setting.) Since your character is relatively physically weak, you're encouraged to use ambushes and sniping. Still, you can render entire sections of The City devoid of human life.
Or, you can knock people unconscious with your blackjack or gas arrows, then drag the unconscious NPC to the nearest body of water and throw them in. Not only will they drown, you will actually get to hear them choking and gasping frantically for breath as they expire.
On some levels, you can throw their unconscious bodies into lava!
There's a lot of lava in the The Dark Project mission "The Lost City." And in Thief Gold, there's wizards from the Mage's Guild running around down there. And the designers didn't add a 'Don't Kill Anyone' for the updated mission. Have fun!
Mark Of The Ninja: Lure enemies into painful traps. Poison them and watch them kill themselves after a psychotic delusion where they'll also kill their allies. Taunt and terrorize a guard, then murder him, leave him hanging from a chain, lure another pair of guards into the area, throw some flesh-eating beetles at the corpse, watch them shine their light on the body, scream in panic, guard in back kills guard in front, you taunt the surviving guard then pop out in front and murder him while he screams in terror. It's all purely optional; you can have a total body count of 3 for the entire game if you so choose, although many of the more imaginative ways to kill the enemies unlock various achievements and terrorizing and distracting enemies before killing them gives you bonus points.
The Special Edition DLC adds the ability to kill enemies with a special kind of mushroom that also infects their corpses, perioidically making a loud noise to lure in other enemies and explodes to poison them as well if they come too close to investigate.
Shoot to wound in Sniper Elite and then shoot (preferably to wound) anybody who comes to help him. He'll die within a few minutes without any treatment. You can practice your grenade skills, run away and shoot him again for bonus sniper points, or just watch.
The sequel encourages headshot and particularly spectacular long-range kills by showing you in x-ray vision what happens to the mook's internal organs when a rifle bullet blasts through them, replete with suitable cracking and squelching sounds.
Dishonored is a game you can complete without killing anybody. It's also a game in which you can summon swarms of rats to eat people alive, rip enemies to shreds with trip mines made of razor blades, toss an unconscious man into a dumpster along with an active grenade then slam the lid shut...
You can sneak over to the control panels of arc pylons and Walls of Light and rewire them. Once done, you can intentionally trip an alarm, which will send guards running at you...only for their own security systems to fry them alive.
Sleep darts are a fantastic way to avoid killing people. They're also a fantastic way to knock someone out so you can feed him to rats, throw him off a building, drown him, slash his throat in his sleep...