Kill whoever you want as an AP, but realize what the deaths mean.

Killing the weak may be good RP for you, but realize that a weak soul does not give you much to work with for your weapon. A hero warrior who's never PKed in their life will not give you anything. For whatever reason, they lost PK fights, so they're worthless to you.

Killing a paladin may or may not be good RP for your AP, but again realize that unless that paladin actively slays evil and wins, again they're worthless to you.

Killing an assassin that has a ton of kills and few deaths under his belt will give you a strong soul for your weapon. What AP really cares if other evils survive? Very few APs would care if an assassin lives or dies, but if that assassin helps improve your personal weapon, kill them. Kill the strong for a strong weapon, kill the weak because you think they suck.