An NPC I killed earlier in the game just showed back up alive and well. Makes their statements about killing anyone pretty meaningless if they just go and do that 

There's a glitch with companion NPCs where they enter a quasi-dead state, or bifurcate into two versions of themselves, one of which can die and trigger the former glitch. This sounds, to me, like that known issue. I haven't heard of it happening to any other named NPC, although generic NPCs do repopulate areas emptied by the player on a rampage as I understand it.

The statements about being able to kill NPCs is largely a quest related one, a lot of games mark quest related NPCs as unkillable or just force the player to put away weapons in quest hubs and the like. The idea that you can still complete quests or indeed the game seemed to be the focus in those statements that I read.