Cynthia was his legitimate wife, and if by chance she became pregnant with his child, her hopes of marrying him would be completely dashed. Even if he divorced his wife and married her, things would never be the same.
Lucca had once believed that he would never be interested in that woman, his nominal wife. But on the morning after their wedding night, she had noticed the deep, unmistakable marks on his neck, scratches that clearly showed he had been with her—and in a forceful manner, no less.
That was when she realized the severity of the situation. In their past, it was always the women who tried to please him, but given his pride, he would never stoop to such low, degrading behavior. Yet he had done so, and it was with that woman. That's why she lost control and resorted to drastic measures.