its cuz his brother is manipulating him using incest and later when he visits the home with duke he will find ou

lan was born to a southern mom who was played by his northern dad. He was sold into slavery
(apparently sexual slavery) to Michele's dad. He eventually was sent to the North, where he was abused by his paternal family as well. Now he's out for revenge. He arranged this marriage as revenge and intended to kill Michele, but they fell for each other and had a happy ending.
I don't remember if the downfall of Michele's family was his doing, but he did get revenge on his own family. Michele doesn't remember him. His brother does, and will try to abuse lan when he goes visit, but Michele protects him.