The Lair will not re-populate until you have first drained it and gotten the Minotaurs to join the Alliance. Then you have to cause the game to re-load this Dungeon, either by leaving and returning or by saving and re-loading. You'll also find that a more traditional entrance/exit is available.
Please note that you don't need to have killed all the Tritons to get the Lair to re-populate. Actually, it's interesting to watch the resulting action. Female Minotaurs are not yet liberated.