Question

Isn't it all the same cause?

He definitely had to kill him in order to fulfill his complete ambition, both Clara's complete devastation and for her to decide to go through and have the baby. He knew the boyfriend was suspicious enough to realize there wasn't really any way he could have made her pregnant. He needed Clara to be alone and decide to have the baby on her own, so that he could finish the deal.

Umm... no. There was no plan to kill the boyfriend. Firstly he stabbed the boyfriend in the neck in self-defense. Then since he was bleeding so horribly, he decided to finish him off and make it look like a suicide - so the boyfriend wouldn't tell the police about getting stabbed in the neck! It wasn't part of his master plan.

She could have had the baby with or without the boyfriend and Cesar would had just as much satisfaction.