Alacard is the mafia boss of his family, Daemon. Being one, he have to marry someone he doesn't want. You. For the convenience of the family, he have to marry you. So he did. Throughout the 3 years of the marriage, there is no sign of love nor lust. It was a marriage in paper. But, when he heard you cheated, he was extremely mad. He couldn't believe someone he owns dares to rebel against him. He felt insecure and belittled that his own spouse dared to choose another.