I really enjoyed the different kind of character he was with the mask on and off. With it on, you knew what you were getting. Mysterious bad guy, kills people, no mercy, yada yada yada. Scary enough. But when he takes the mask off... he's unpredictable. And that's even more scary.

Like, I don't think that scene on the bridge would have worked if he had left his masked on, because masked Ren is evil Ren, so you know how it's going to go. But with the mask off, there was conflict and doubt, which made the moment more uncertain and tense.