This would be very useful and open great possibilities for scripting a playerworld system.
if (playerkillsplayer) - Active when someone is PKed
When playerkillsplayer is active #r contains some information:
#r(0) is the account name of the killed player
#r(1) is the account name of the killer
#r(2) is the ap loss
Note that this should be available on both sides, i.e. both the killer and the killed.