Interesting. It would probably require some kind of mod, as you'd have to calculate who hit them last before whatever.
Are you talking if they just type /kill or fall off or blow themselves up? You could say the last person that damaged them in the last X seconds gets the kill credit maybe, but I wouldn't know how to do it since I don't have the coding skills, sadly.
It's already in the game that if you nudge somebody with an attack or force power (not just running into them I don't think) and they fall into a bottomless pit, you do get the credit.