He starts to like them, which makes the betrayal even more painful.
The more he did something when he doesn't want to, the more power it gives him.
Personal problem with the taboo as represented in the story though.
Seeing the comment section, readers seems to expect the taboo related to sexuality, such as sex with loli or own parent/sibling. Meanwhile, Alex proposed things like patricide or massacre of his kin (on both human and demon side). While both are considered morally wrong, and people would normally feel aversion towards it...
When presented in the story, the
guilt stood out more. It just... felt not enough to convey the "taboo" concept itself.
Sooooooo does someone have a theory about how his revenge will play out ? Saying that now because he just said that he grew feelings for a bunch of demons but he still want to destroy the kingdom and it's obvious that he'll have to get rid of them in order to do his revenge (also theories not spoilers please).
Lets just say there's not much to spoil about that yet.