So what's actually the main reason on why Kyuho wanted to leave Jeho group? His father is ...

I think at first it’s just to rebel like what you said but after he met Kangha, he wanted to be with Kangha. It’s because people who he loves won’t be happy cuz of how well known and powerful Jeho group is, also how messed up their family issue was. Like at the beginning, there was a flashback of Kyuho on his sister’s wedding day talking about how she had to give up her love to marry someone else for the family’s sake or something like that.