Hanako-kun spoilers.

*HUGE SPOILERS AHEAD*

Hanako's real name is Amane, and he had a twin brother named Tsukasa. The reason why Amane is one of the seven mysteries is because is a punishment for killing his brother. But Tsukasa isn't a saint either. When they were alive he constantly abused and tortured Amane just because, and considering how Amane/Hanako is completely terrified of Tsukasa when he sees him again as a ghost, Tsukasa being extremely clingy and "affective" towards Hanako, and the fact that Hanako is portrayed in suggestive poses in multiple chapters (some of them with hands touching him, like the one in the anime), it's possible that Tsukasa even abused sexually of his brother (which could explain why Hanako killed him).