He isn't in love, he is just obsessed. But since he isn't actually capable of falling in love, this is as close as he'll ever get to being in love.
Why is he not capable of falling in love?
He is basically a psychopath in the novel. The fictional kind anyway. Or maybe more of a sociopath? Anyway, he clearly is ment to have ASPD, even if given the setting, it isn't explicitly stated. And since he has the power to act as he wishes with no fear of consequences, he is just to going wild, doing what he wants and always getting, what he wants and acting out if he doesn't get it.
And Kay suffers for it.
If that's not black magic I don't know what is. Also I refuse to believe this mf is in love but wtv I will eat this shit up *swallows* ┑(￣Д ￣)┍