Dwarf Fortress's adventure mode. Not only can you indiscriminately kill (although the guard will come down heavily on you for that one), but there's a fun way to kill people without getting into trouble:
1. Find a dwarf, elf, human, or other friendly standing next to a tree. 2. Set the tree on fire. 3. Friendly dies, because DF is very, very bad when it comes to fire (unless you're after the laughs, in which case it handles fire perfectly).
The newest versions of Dwarf Fortress allow players to specify their attacks in great detail in adventure mode, including the type of attack and the body part to target. This includes, but is not limited to, grappling and breaking limbs and joints, hacking and slashing with edged weapons, bashing with blunt weapons and shields, punching, kicking, scratching, and biting. Given how the game generally doesn't use a hit point system and bodily damage is realistically simulated, players could potentially break an enemy's every joint in every limb, gouge out both eyes, bite off both ears and the nose, and cut open the enemy's torso so the guts spill out. The guts may, if desired, also be ripped out entirely. Or grabbed to swing the enemy around. And this is all in one fight, against any NPC you choose.
And the game mechanics also discourage mercy since every hit gives you more experience, thus slow deaths of a thousand cuts (sometimes literally a thousand cuts) will increase your skills in a much faster and safer way, since after a while they either fall unconscious from the pain (though no experience is given while they're unconscious) or are so crippled they can't get a hit on you.
FTL: Faster Than Light. How can you best screw over your enemy... well, you could disable their shields in an asteroid field and let flying space rocks tear their ship apart... you could start a fire that spreads through the ship and burns the crew... or you could always vent your own ship when enemies try to board it and suffocate them to death...
There's an achievement for disabling an enemy ship's life-support until they all asphyxiate.
There's also an achievement for having every inhabitable part of an enemy ship on fire at the same time.
The game allows you to murder innocent NPCs. Certain types of NPCs such as small children, old men and beggars don't give any karma penalty when killed, so you can go around slaughtering kids and still be considered a saint. And with NPCs that do give a karma penalty, the penalty is usually small enough that you could just deliver a few items and be in better standing that when you started.
You can also breed and farm any kind of pet/companion, including humans. So you can farm little girls for their meat.
You can find a Sandbag if you're lucky enough, which you can tie an NPC to, allowing you to attack them infinitely without killing them. And you don't even gain experience from it, so the cruel fun of torturing someone is your only benefit from using this item.
If you accept the tiny girl's quest to bring back her cute dog, you can forfeit to reach the bottom floor of the dungeon in time and arrive later instead. This will generate only the cute dog's corpse. Which you can then pick up, bring it to the tiny girl, and eat it right before her eyes.
There is a nice, sad baby water dragon called Blup in the village of Terinyo, looking for his mother. You can find her, kill her and take all of her treasure, then return to Blup and tell him you found her, and watch him fly off to meet her.
Beggars are happy when you give them food. Corpses are food. So you can kill a beggar and feed his corpse to another beggar. Bonus cruelty points for poisoning the corpse first.