So we don't know the predicament that lead to the arranged marriage, but it appears there are some circumstances.
Instead of doing this marriage out of love or duty it is for the sake of her family. He admitted up front that he didn't love her but was willing to try. I can't help but think when both parents died what tied him down was gone and he wanted out but didn't want to be the one to suggest it. And then one day he got fed up at being subtle, and decided to act out the villain to make a quick clean cut.
I don't justify his actions, but i would like to know more about the circumstances that changed his personality so much. what made him turn from level headed and dedicated, to callus, ignorant, and naive.