What did kuro-kun do to make him a devil?
Brcause they had sex until death do them apart?
No... he inknowingly "broke" a lot of womens girls for being so desirable yet so out of reach, and for how they cried their hearts out when he died.
Well, karma's supposed to work that way, so it makes sense...
*unknowingly broke a lot of women's hearts.
That's because In a sense, angel-dude sacrificed a lot of his own comfort and happiness by dating all those women he didn't love, instead of being with guy he liked, while demon-guy while popular never really did sacrifice much of his own comfort for the sake of anybody else, except for the angel-guy that time when he spent 3 days helping him.
So.., in this story if you are a good person, but the outcome of those actions result in bad things then you will be a devil..., but if your intentions are more devilish, but the outcome of them are good then you will be an angel.
Damn.