How does Cassy feel about her new master? Why does he sell her and their children?

In Chapter 33, Cassy shares her hatred of Legree with Tom. She tells Tom that she is Legree's mistress. Cassy told Tom that her first master was inder than Legree and had promised to free her. Unfortunately, he wasn't able to free her before he dired, and their two children were sold away to cover gambling debts.