He was mistreated and his pride was stepped on. He remembered how bitter it felt and how he didn't want to be there any longer. 

The only reason that has kept him there was the group members. They were the only ones who treated him like a family, especially Heechul. He stayed and endured the hardships for them because he had loved them too much. 

But the unfairness didn't stop, it got even worse. They thought they've gotten the best of him, they thought he would never dare to leave. But he eventually did, and he stood even taller. He was now alone, but that didn't mean he was lonely because he could still contact the members occasionally.

Hangeng was actually thankful, because now he's finally thinking for himself. And he's become stronger, better.