Basically Serizawa is being violent with Ukyo (even though he loves him) so that if ever they got caught or Ukyo exposed him, he will be seen as the bad guy and Ukyo the victim, and in order to protect him (from consequences like criminal record) his father will forgive Ukyo's father debt, and therefore will make Ukyo free from him.
Ukyo understand this, but will not expose Serizawa because he loves him too.
Basically Serizawa is being violent with Ukyo (even though he loves him) so that if ever they got caught or Ukyo exposed him, he will be seen as the bad guy and Ukyo the victim, and in order to protect him (from consequences like criminal record) his father will forgive Ukyo's father debt, and therefore will make Ukyo free from him.
Ukyo understand this, but will not expose Serizawa because he loves him too.
Serizawa is a dick but not so much.