Playing a thief is unnecessarily violent. I like playing the thief and even some of the discussion from the thieves guild suggest that killing is not desirable. However, you can't just knock people out. I think it would be awesome to introduce black jacks into the game. Just like how a back stab can be instantly fatal, the black jack would simply knock out the victim instead of killing them. To me that would be much more desirable. If I wanted to kill everyone I would join the Dark Brotherhood. I just want to sneak around, steal stuff, and get out. It is really sad that death is the only option in this game.
I haven't really tried the thief approach yet, as I'm playing more of a balanced warrier style, but have you tried some paralyze spells or potions? I was going to play an archer/thief on my next playtrhough, and I was really hoping I could just stock up on paralyze poisions to drop the occasional enemy that might witness me, but without killing them. Is that not possible?