All Fallout games:
- Fallout: no kills required. enemies can be KO'd by hand-to-hand, or beaten until they flee
- Fallout 2: no kills required, but fuck knows how you're gonna get through the tanker basement. same non-lethal method as above
- Fallout 3: one kill required, a radroach in the tutorial. you also have to clear the super mutants from the water thing, but you can do that indirectly. no way to KO enemies, so you have to use stealth
- Fallout: New Vegas: no kills required, but you typically do have to incapacitate Mr House. same as above, stealth required. there is actually a knockout mechanic, but it's fucking terrible
All Elder Scrolls games:
- Arena: one indirect kill required right at the end, otherwise you can get through the whole game with spell reflection and calm humanoid. the start of the game will be insanely, unbelievably tough though, since your only method of gaining XP will be to make a very weak reflect spell that you can cast at level 1, and grind against uncommonly-spawning magic using enemies like the snow wolves
- Daggerfall: you have to kill a werewolf and a zombie messenger, but I think that's all. both can theoretically be killed indirectly. otherwise, calm and reflect spells. there are also linguistics skills which sometimes let you automatically talk enemies down, either pacifying them or making them join you and defend you
- Morrowind: several kills needed, but they can all be indirect (or you can just cast Levitate 30 seconds on yourself, Levitate Other for 29 seconds on the target, then walk directly upwards into the sky while they chase you until their spell wears off and they fall to their death). you could probably get quite far using reflect and calm spells again, or Command Creature/Command Humanoid
- Oblivion: can't remember, I don't think there's any mandated kills. same as above, you'll have to use non-lethal spells
- Skyrim: forgot how the main plot works, but most sidequests and dungeons can be completed without actually killing anyone, especially if you abuse the borked stealth system
Deus Ex games:
- Deus Ex: one non-combat kill needed ("take your best shot, flatlander woman"). The game really doesn't care what you do after the UNATCO missions, though. non-lethal methods are baton, prod and tranq bow
- Deus Ex HR: one kill needed, the Rihanna cyborg. You have to fight the big mech guy too but he technically kills himself, though there's no difference from a gameplay standpoint. non-lethal methods are stun gun and tranq rifle, but also running up to people and pressing Q to watch a 3-hour takedown cutscene
Age of Decadence: no kills needed, but everyone's a bastard so you'll probably lead to someone else's death, either by your actions or by your failure to act. combat is avoided through skill checks in dialogue
Alpha Protocol: no kills needed right up until the stupid final level, which requires you to shoot down a helicopter and kill one guy. combat is avoided by stealth and non-lethal knockouts
Mount & Blade: you can use blunt weapons to KO enemies. You can theoretically do the whole game like this, but you'd have to make an army entirely out of Manhunters/Slave Drivers (the only two units to use blunt IIRC) and somehow take on hundreds of heavily armoured, highly trained elite knights with them.
I liked Expeditions Vikings but I seem to remember the non-lethal stuff being very half-implemented. You'd KO a group of people and the game would consider them dead, for example, and your choice was very very rarely commented on, if ever.
