I just wondering if things in the watseland can kill npcs related to a quest similar to the merchants on fallout 3? If so this will be rather annoying doing a quest to later find out after serching for hours to find he/she is lying dead. As every NPC but one is killable does this also include something like a Giant Radscorpion killing them when you not around?

Yes.

I was walking toward New Vegas just in time to see Victor the robot get blown up by one of those giant oversized crab things that live in water. No more victor

Most quest related NPCs don't hang out near radroach nests.

*giant radscorpion* sorry for saying radroach lol was wishfull thinking, at least I hope the main quest people don't decide to be stupid and die.