I am really loving this season, and this episode was hilarious! I was glad that Merlin got to be evil/murderous Merlin... that was huge fun...

I thought it made sense that Merlin hesitated to use magic in front of Morganna: unless he was certain of winning and killing her, he couldn't afford to have her know that he could do magic (or suspect that he is Emrys). We could see during the fight between Emrys and Morganna that they are pretty evenly matched (ie it might have been impossible for Merlin to win when he was in chains, and maybe he knew it). Anyway it makes sense to the story arc that they need Morganna to continue to live and be ignorant of Merlin's power.