I've actually led a pacifist RPG! In Pathfinder, of all things! See, the players were a police force (of sorts) and each case (which I made up to be an episode of a show, complete with credits and all that) had stated goals one of which was zero casualties. As a result, one case I can recall had exactly one attack roll rolled and that was against an inanimate object.
The players are generally quite fond of a good dungeon crawl but they enjoyed this, especially, I think, because they were armed (heavily, too) but had cause to exercise restraint. It was a fun challenge and the campaign is still fondly remembered.
 There was a definite X-files vibe to the proceedings.
 Which was in-character for the not particularly nice regime they were working for for tedious political reasons.
D&D 3.5e actually had pacifism as a rule: The Book of Exalted Deeds had oaths you could take which compelled you to certain conduct but gave rewards in turn. I believe pacifism was one of them.
Even mainstream games can be played quite flexibly and even if you keep to the D&D school (of what are, latterly, essentially miniatures wargames) you can explore non-standard modes of play. Pacifism is completely compatible with roleplaying. And if you choose more experimental games this only becomes easier. Spirit of the Century offers a modeling of social conflict using a similar ruleset to its combat which means you can absolutely have games where the big fight at the end is actually a masked ball the end boss is the dowager duchess and her odious political views. And you don't need to do any serious adapting to the rules since the players already can make non-combat characters with considerable ease.
As for they die/you die... I think I disagree. I mean, first of course, there's weird experimental stuff like Dogs in the Vineyard which is explicitly built to avoid this sort of thing by using this weird poker-bid based conflict resolution system. But secondly, even as hoary a system as AD&D 2nd edition features morale scores for most monsters (certain undead and golems never lost morale, of course, because they were mindless) and fights were, if played correctly, not to the death but to the running away. Players, too, often had to leg it since AD&D was comically unforgiving.
And lastly, the idea of violence having consequences, that, too, is a very common thing. Played straight, murder, even in self defense, is supposed to ding your sanity meter in Call of Cthulhu, and in Unknown Armies responding with violence is the absolute worst thing you can do.
I guess what I'm trying to say is that pacifism in tabletop gaming is possible, has been done before, and games that take violence semi-seriously (they are games after all) do exist. And it can be fun, too. I will admit, looking at the above I feel a bit... cold, since it strikes me as preachy. I much prefer the Unknown Armies approach where violence exists is absolutely effective and is as terrible an idea as it is in reality. That said, Unknown Armies is, on occasion, a very grim experience.
(And yes, I am a nerd. )