@Honeybeaubie
Dion is not a masochist. You are mistaking cause and effect here. He literally doesn't care about the physical side of the slap. However he is deeply satisfied with the fact that Ana had the urge to slap him. He wants her to be distressed, angry, miserable, desperate. The whole family is big on physical torture, but for Dion the physical is just a tool. He excels at it, but doesn't abuse nor care about it much. And using it for torture is just so unsavory to him.
No, it seems to me that for Dion the best fun is making people feel raw negative emotions. He is basically a sadistic troll that knows your secrets. Such a deep character compared to his psychotic one-dimensional mother. Of course for now he's depicted as an evil Mary Sue, so it's a pretty picture but somehow lacking - lacking weaknesses to be specific. He might be even in love with Ana in his own twisted, possessive way (similarly to the younger brother). Remember - it's a harem setting