I have theories.
Will's mom died giving birth to him. "Never knew her."
Will's father then began to resent his son and became an abusive alcoholic.
Will having an abusive father is the reason he developed the ability to read people so well. Fear of his father made it so he learned to recognize patterns and subtle cues, so as to prepare/prevent.
It's also why he has identity issues -- he mimicked his father because it was only then that his father didn't punish him.
It's why he is obsessed with being a good father, and also why he has dark impulses.
It's why he's terrified of being bad -- he doesn't want to become his father.
It's partly why he becomes so attached to Hannibal. I imagine he feels similarly about Hannibal as he did for his father.
Possibly killed his father in self-defense.
