[AndMetal]

I think you're looking for $npc->Kill().

Example:

sub EVENT_SAY {
	if ($text=~/kill yourself/i) {
		quest::say("As you wish, oh great one");
		$npc->Kill();
	}
}

Found here: QuestObjects (Wiki).

Something interesting about the quest system is that, basically, the quest:: commands are wrappers for the commands listed in the above Wiki article. For example, you can make an NPC say something using $npc->Say(), although that one in particular takes an additional option (format).