He hates himself for loving SH. Bc every anger, remorse and sadness he felt during those 10 years feel so easily and quickly distant when he's with SH. He never really wanted revenge on SH but to die by his hands since he is dying already, and if there's a small chance, to be remembered by SH this time around bc SH forgot him so easily before.
Love and hate are 2 sides of the same coin. He had to use whatever energy he had in those 10 years to focus on something to live, so he "hated" SH but in reality, he yearned for him and even made sure SH will be fine and unaffected by the Kujo clan after he dies. He's always loved SH but afraid to admit it completely bc he shouldn't feel this way, otherwise all his built up anger feels so petty at this point.
Not to mention, the trauma of being a rape victim, he doesn't know if his body is reacting to SH bc he has feelings for him or bc his body was conditioned for it, which is really fucked up. I like how KG toned down the rape in this series and really goes through the psyche of the victim. The hatred and disgust they feel about themselves when they're the victim.
Idk how to describe but I feel like YT hate SH as a cope for himself. Deep down he hate himself more than anything and he’s still in love with SH. But if he admitted it or if he forgives SH it would be “too easy” for what he went through. And thus he would hate himself even more.
So what he thinks he wants is to die by SH hand and leave a scar in SH’s heart. And for his beliefs SH need to be a jerk who deserves his revenge and he need to hate SH.
Now that SH apologized sincerely and treated Yuta better, YT realized that he would forgive SH and SH is not a jerk who deserved his revenge anymore? That’s why he said he hate it.