Mutual dislike
There is a saying that villains aren't born but are a product of their environment. The way they live, their families and each negative word they are constantly bombarded with day in and day out breaks them apart and reshapes them into the person they are.
As for little Rui, he was already born with a predisposition to become a diabolical black-hearted person. His aether was an empty shell with no ounce of seraphic energy but the universe had given him a loophole. He could gain the power that he so desired by robbing others of their seraphic energy to fill in his empty core.
And for him to commit such a heinous crime he had to be furthest away from a warm-hearted kind person. Within his core, he had a vile seed that could erase the goodness in him and the moment his mother died the darkness spread like cancer resulting in the unfeeling lunatic who bred with multiple women so he could murder his offspring for power.