For many, many, many crimes, police decline to send a case off to prosecutors or prosecutors decline to take a case, because the whole thing is too murky and confusing, and the participants decide they aren't happy with the process or its probable outcome or whatever. When somebody actually dies because someone takes a weapon and kills them, however, it is a lot harder to just say, meh. Too complicated. I mean, that _does_ happen, but if the perpetrator is identifiable, murkiness is more likely to lead to prosecution rather than abandonment of the case. (If the perpetrator is uncertain, and the victim sufficiently unsympathetic from a police perspective, I suspect abandonment happens a fair amount.)
It's a little bit tempting to say, you know, when you get one of these intrafamily disputes, and there's clear history of the dead victim damaging the killer, let's just not even bother. Basically, if you have a documented history of beating someone up, and they come back and kill you, it's not a crime. You asked for it, essentially. That would seem to put the responsibility for not beating people up or leaving if the temptation to violence is too strong firmly on the aggressor, rather than the victim.
I'm sure there's a huge flaw with this idea. I just can't think of it at the moment. And I figure if we've written statute to capture Stand Your Ground and Castle, there's no obvious reason you couldn't write statute to capture Justifiable.
ETA: Hey, someone else came close!
This would create a new category below involuntary manslaughter. Not quite the pass that I would give (justifiable, basically).