"You've been at court all this time, at Arthur's side. How you've managed to deceive him... I am impressed, Merlin. Perhaps we're more alike than you think."
That is the only stroke of interesting writing Julian Jones showed in the entire script of 4x13 in my opinion.
Merlin the master manipulator. Just like Agravaine.
It doesn't matter that Agravaine was evil. It doesn't matter that Merlin is good. They are both deceiving, lying to and manipulating Arthur for their own ends. They are both essentially betraying him. And the more Merlin keeps his magic secret, the more he's no better than Agravaine really. His good intentions fall flat under his questionable means.
Poor Arthur. Poor oblivious, true-hearted, innocent Arthur... A puppet on strings...
Do. Not. Like.