Constance is the one making things difficult. Why can't she love him and marry him so that they will leave the country together. It hurts him so much that Constance couldn't love him. It makes him angry that he loves Jace and not him. 

He doesn't want to hurt her, he doesn't want to lay his hands on her but she pushed him to the wall. He couldn't control his anger when she kept on saying that she would never love him, that she only loves Jace. 

Darren can't help but blame himself for the condition Constance is in. If he hadn't lost his temper, all these wouldn't have happened....