I've kicked for people killing the low karma people.
I don't see why you should be allowed to kill someone (who more than likely is new, THEN decided to read the rules) just because they can't hurt you back. Imagine you'd feel if you messed up and everyone killed you for no reason. And since most people who kill low karma players seem to insinuate that "they can't do anything", let me just point out that is wrong. You get a karma bonus for dealing damage to traitors; you don't neccessarily have to kill them to start working your way back up.
If you want justice for them rdming, talk to a vip or something.
Or if they keep it up report them.
I've found when you say "kreporting" they leave, usually.