Miyako is an extreme sadist.
The reason for why he developed that way isn't really clear, as we don't really know much about his past.
It might have roots in when he met Toma, and bullied him.
Kids often bully the ones they like, because they don't know how to convey the emotion of "love/like" that well.
Even some adults have a hard time conveying emotions of love.
Now his sadistic desires could come from a number of factors.
Maybe it's through simply conditioning, where Miyako might've associated sexual stimulation with a violent/sadistic situation.
It could also have roots in his (Maybe) own experience of anxiety, attachment issues, abandonment etc. Again We don't know.
Furthermore it could just stem from his desire to seek the highest pleasure. Sort of like an adrenaline junkie.
The cause of his sadism could be many things and we only know so much.
It's a very complex subject to discuss, as the reason behind sexual desires, differs so much from person to person.
There might not even be such a deep psychological reason behind it. Personally I get the impression of his sadistic desires to be rooted in his urge to have complete control over Toma.
In most stories i can relate to the character's motivation even if they're not actions i would take. Psychologically this is interesting but I don't t understand the "why" they behave this way at all! Particularly how twisted Miyako is. Wanting to hurt or destroy someone you care about (as opposed to revenge) isn't something I understand. Its not even "if I can't have you, then no one will." Actually,understand Itsuki somewhat. Anybody have deep insights into the characters ?