You know I've been thinking about it and I don't quite get it.
Like, yeah, I understand what happened, but why did he have to leave? I wouldn't have left my little sister to die alone. I would have stayed at least until she died, doesn't that make a bit more sense?
He didn't decide to leave, his father sent him away to avoid the political scandal that his family is in. Notice he's traveling alone which means his parents are still in China dealing with the situation and taking care of Min-Min. She doesn't have to go to school like he does so she won't get attacked in school.
I don't think he had a choice. His father made the ultimate decision to fly Kai to America, I doubt Kai had a say. And besides, Kai was getting bullied in school, and it was very painful for him to be there as well Lose-lose situation. There's no way to win. I thought this story was gonna have a happy ending D: