1) Aren't forgetting that PK doesn't actually exist? I can't remember the last time someone was PKed outside the arena who wasn't either AFK or so much weaker than their killer that it wasn't worth remembering.
2) Can't we do this already? There are boards for messages. There are people who already put contracts on each other. Nothing ever comes of it of course because it is 100x easier to escape from PK than it is to kill someone.
3) I like the assassin mob idea. I think the bodyguards place in Little Italy used to have an option for putting a contract on another player but either it never worked or was never dangerous enough to bother with. Hiring tier_4 mobs to harass people sounds awesome though. Do that.
4) I don't get it. Do you just want a coded way of doing what we can do already? Or is the idea to have a magnificent reward appear for killing someone, in which case it will be 100x easier to abuse than it is to claim it legitimately and it will probably only be worth the effort if you're abusing it anyway.