Like the Babe calling his shot or Arnold saying hasta la vista. Do I have your attention? Good. Now watch this...I originally thought the apprentice thing too but I dismissed it because this would have taken place so early in Maul's training that I doubt he'd even started to think about killing Sidious yet. And I never insisted that it was a moment of compassion, rather that the pause was set up to give the moment gravitas. Not that he was hesitant to kill her, but that killing her was of particular significance to his apprenticeship.