Honestly, he is. I felt bad for him because of how she treated him, but it's obvious that he's got some screws loose.
Spoiler
I just reread this part and it makes it seem as if he was brainwashed, but it was his main intention this whole time to do this. Yvvones brainwashing doesn't change his goal she just amplifies it a bit. Whenever it shows that he's "brainwashed" it's Yvonne telling him that she'll help him acquire Penelope. He doesn't care for Yvonne at all. He even tells Penelope that he'll kill Yvonne if it makes her happy. He starts a rebellion just so he can become ruler and kill the crown prince so that he doesn't marry Penelope. From my point of view he probably would have done all this regardless if Yvonne was there or not.
Is he planning a rebellion? But why would he get his fellow people killed? What is he planning? ヽ(`Д´)ノ