This should depend upon the reputation of the dead person, as well as the skills and reputation of the witness and player. So if you attack a robber guards and good people with good skills should help, good NPC commoners should run for help, neutral folk should be neutral, and bad folk should attack you or (if their skills and level are not commensurate to yours) run for help.

call you an nwah before slaying you with a chiltin dagger.

Assuming we use stuff from past games, I would use a combination of responsibility and aggression stats, faction relation, and skill-check. You don't need to go more in-depth than that, especially considering how complex it would make things. Plus, most of those variables could be re-used in other AI situations.

I've got one: If the guards just helped you kill someone attacking you (think mythic dawn assassins), they shouldn't immediately bend down and say "THIS MAN'S BEEN MURDERED!" It was rather funny, but still.