His "pregnant" ex-fiance? I thought he was sterile? Or was she pregnant by another man?
Did he ever say why doctors told him he was sterile in the first place?
It honestly sounds like he was looking for an excuse to leave. He brought up an abusive ex, compared her behavior to yours, and then got upset when you didn't like it. He sabotaged his own relationship and probably on purpose. If I were you, I would cease contact with him until he either...
1. Bucks up and comes back. In which case, there would be some serious house rules in place as to what will and won't be tolerated. Mentioning abusive ex's and throwing a tantrum and walking out will not.
2. Contacts you for a paternity test and child support/visitation.
Don't chase after this man. Don't contact him. Don't ask yourself what you did wrong or why he's doing this. He did this, and he has his reasons (even if you don't understand them - who really knows how the male mind works?).
Concentrate on yourself and your baby. Only worry about him in regards to supporting your child. If he decides he wants a relationship with you, he's going to need to earn your trust back and prove that he deserves to be in your life. You deserve nothing less.