How about a possibility of talking humanoid monsters into neutrality using Charisma, with effect similar to the Calm Monster spell. How it would work: upon chatting to hostile humanoid, there will be Ch/10 % base chance that the monster will turn neutral (that means with Ch=99 about 10% chance). It would work only if the PC has not yet attacked the monster, and of course not for uniques, probably also not vaults. The chance would be improved to (Ch+Ap)/10 % if the monster is of opposite gender. Then add/substract 0.1% per level difference between the monster and PC (higher level monsters should be harder to turn neutral). Add 1% if the monster is the same race as the PC. Substract 1% or more if the races hate each other (dwarven PC trying to talk an orc into neutrality should be harder). Add 1% if they are the same alignment and not chaotic.
If the check succeeds, message: "You succeded in persuading -foo- to be neutral!" and the PC would also get fraction xp (like for saccing), and +1 alignment. Killing the monster later would yield no xp, and appropriate alignment drop. If the check fails, standard message is displayed and subsequent attempts with the same monster are automatic fail.