Alter Aeon Rules of Conduct

Rule 3.6 - Thou shalt not kill steal

Kill stealing happens when one player is fighting some monster, and
another player shows up unannounced, joins the battle, and lands the
killing blow.  When this happens, the experience from the monster
goes to the wrong player.

Kill stealing is considered a minor offense, and there are cases where
it needs to be done and other cases where it simply can't be avoided.

You should attempt to avoid kill stealing if at all possible, if only
because it's the polite thing to do.