Sadism wasn't the right word to use though. It implies that the girl derives pleasure through instigating pain. That is, however, not the case. Tsunderes don't derive pleasure from the tsun- part. Indeed, the scene after a tsun tsun outbreak is usually the girl talking to herself and wondering why she can't act normally. I couldn't think of a word to describe that situation.
Masochist works because you'd need to be one to continue that relationship, regardless of the girl's intentions.