Does anyone know of good variants? I'm not ready to simply do milestone progression. I like the spirit of the experience system for Dungeon World but adapting it seems like a lot of work.
In my game, you get as much XP for converting something to be a loyal ally as you do for hacking it to pieces.
You also get a lesser amount of XP for evading something or chasing it away.
None of that I think lies outside the notion of defeating or conquering a foe so if it is a variant, it's only a mild variant.
I'm not convinced that the XP system is what causes players to act like murder hobos; they acted like murder hobos when we played a different system as well. I think players often enjoy acting like murder hobos.
There is also the role of the GM in this. In my experience as a GM, it's very hard to avoid introducing monsters that don't ambush players and fight to the death. If you try to use monsters as more optional encounters, and the players don't act like murder hobos, there is a tendency for the encounter to be something of a time waster that didn't add much to the story. There might be some challenge in evading monsters that didn't particularly want to fight you in the first place, but its not a lot. And there might be some story in evading conflict with things that weren't particularly hostile in the first place, but it's not a lot. So if you put a monster on stage, there are very few functional roles other than antagonist. And even if killing the thing doesn't earn a lot of XP, there are very few more functional responses most of the time than killing something and taking its stuff, because stuff.
This often leads to the approach, "We're friendly, but if you won't help us, then we might as well kill you."
Add to this problem that some GMs often load up NPCs with lots of nice stuff either the make them 'cool' or to make them worthy challenges, and killing whomever you come across can often be not only fun but profitable.
If you want to blame this on system, I think you can go much deeper than simply blood = XP. I think the basic problem is that D&D makes combat one of the most fun if not the most fun things you can do in the game.