Something to ponder on, but not intended as a definitive solution by all means.
The way I see it is that the people you really only kill are the people in the campaign. Everyone outside of a mission? That's just you goofing around (although I appreciate that Michael speaks about stuff you've done during his therapy sessions, but I'm not too sure) and doesn't tie in with the world as much.
It's also worth noting that (until V), it's unclear if each character exists on an official, national database, level.