The Master Assassin offers you the Bell of Opening if you will leave him in peace and let him keep the Master Key. If the player wants to play a chaotic good rogue, they are able to go back and kill the Master of Thieves (without immediately being kicked out of the quest), after which the Master Assassin will give you the Master Key.

The Master Assassin has an instakill attack (though not unpreventable; the player should be able to prepare and protect themselves some way).