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.
Lets just say there's not much to spoil about that yet.