Yeah, blackmail makes a lot of sense. It would explain why he was called a tyrant despite being a good person at heart. He'd be ruling the kingdom with an iron fist if that meant keeping Ginny alive. Perhaps at the end he actually wanted to lose his humanity and free will, maybe to forget all the suffering he went through, maybe as a form of self-punishment for the suffering he caused to others as a king. The fact that he had a daughter and that she got to know him as he was before becoming an emotionless robot only makes the whole thing more tragic.