Delilah's death.
It's what kick-starts his determination to actively become evil.
Despite being treated badly by his biological family, her good presence restored his trust in the goodness of people. He had only been unlucky by being born in a broken family, one that discriminated against him, all because he was unlikely to become a Hybrid.
Perhaps in this new family of his, it will be different—things will be different.
Since he learned how to be loved and cared for from Delilah, things should go differently.
But he was wrong.
Delilah was taken away unfairly, leaving him to realize that goodness couldn't survive in this world.
He realized the darkness in this world could swallow everything other than itself.
Only the evil could fester and grow stronger.
All this time, he had blamed the world for Delilah's unfair death.
It's the reason that drove him to grow stronger as quickly as possible so that he could change the world.
Change it for the better.
