There's a lot of interesting thoughts here but I'm gonna throw a tangent out just to highlight how weird murder is to our brains: video games. They often engage in what's called ludonarrative dissonance - when the narrative is ludicrous but the viewer expects it. TV shows do it a lot too.
You'll have characters go on murderous rampages of countless nameless baddies but suddenly they have enough emotion for a cut scene where one of their loved ones is killed. And we expect it.
RickiTarr