It turns out it is not too difficult to finish the game with no kills at all.BasiliskWrangler wrote:"20 kills" was really an arbitrary number. I just set it up like that, and never changed it.
If it turns out that less that 20 is extremely easy to do, I can certainly adjust that challenge for future versions.
I can only add that playing as a pacifist is a lot of fun, as you have to plan carefully and come up with with some creative solutions, especially if you're aiming to finish as many quests as possible. Hats off to Tom, there aren't many RPGs that I know of that actually allow you to stealth your way through the game without killing anyone.
Here's some related information you may or may not find interesting.
There are currently three quests in the game that I believe cannot be completed without killing someone (one due to what I think is a bug), plus one I am not entirely sure of.
Spiders in the Warehouse.
The warehouse won't open until the last spider is dead. It is not too difficult to use strategically placed charm clouds to get them to kill each other, but, ultimately, you will always end up with one left. It could be done if spiders could poison other spiders, but that is not the case and I cannot think of any other solution.
Exterminator
Similar to the spider situation. There are several areas with no creatures other than the boreheads and no means to kill them indirectly.
The Curse of the Prospector
You can charm the rank and file undead in the area but they cannot hurt the Prospector. He can't be made to step in the shock barrier in the southeastern corner of the map, either (because he can walk on thin air). You can lure him all the way back to the Mayor (although it is somewhat difficult), unfortunately, any Mayor's kills count as your own (you even get experience for them). This is probably a bug, though, as it doesn't happen for any other NPC in the game.
In the Belly of a Beetle
I used a rogue character with focus on hiding in shadows, skullduggery, picking locks and alchemy. My only chance to get the petrified wood from the bug was to find the right one, bring its hit points low with a few demon oils or a powder keg, then use a charm cloud on another borehead to finish the first one off. This doesn't work with alchemy. The game informs you that "your casting level is not high enough" to charm a borehead, which nevertheless means it's probably possible to do with the charm spell and enough points in divination (can someone confirm?).
All other quests can be finished without recording a single kill, including those where the actual objective is killing someone or something. This is achieved with the aid of a charm cloud potion/spell or (where applicable) simply weakening the target and luring them to a friendly character who will finish the job. With the current pacifist requirement of less than 20 kills, it is most probably possible to finish all quests, especially if it's possible to charm the boreheads.