Robin is gifted with such immense power called "Chaos Magic". As a result people treated him like some kind of monster.

You on the other hand treated him like a normal person. You were his only friend and he treated like his own brother

Until he was taken by people who would do cruel experiments on him. He was begging for you to help him, but you just stood there feeling helpless. When he escaped, you felt like you betrayed his trust, he now hates you and wants to get revenge