I think he was holding back all along. He loves Yuri way to much, so he holds back all of his feelings. He does not want to steep over the line. As if he knows that if they sleep together, he wants Yuri for himself. He knew he would be territorial and nobody is to touch him. As the VIP said "Are you afraid? That once you embrace him, he will push you towards insanity".
But yeah, I agree the ending was kind of crappy. It was to damn sexy and intense the first chapters, but chapter 3 and 4 is a bit rushed. I miss the longwaited-craving sex, and not some drug to start them off. Wished it to be a more BAM! kinda ending. Yeah, it was kind of sweet, but not like it should be.
Adam wasn't seeing Yuriy. He was seeing Yuriya (the woman who exploited and controlled him, and who Adam murdered). When Yuriy says he'll take his mother's place, Adam continued his vengeful control, his hatred for Yuriya, on the son. (The names are alike for a reason). And he also has growing desire for Yuriy, that's twisted by his hate for Yuriya.
It isn't until Adam sees YURIY, not Yuriya, YURIY, because the drug has lowered his inhibitions and increased his libido, that he starts to break free of the curse of Yuriya, and then Yuriy can break free of the curse, too (both of them have been under a spell of sorts).
When Adam regains himself, and he sees Yuriy as Yuriy, they can express real love (healthy love), not the twisted thing. When the shoes stop having hold of them, they are free to be who they are with each other. The mother is fully dead and BURIED now (hence we see the grave of the mother, she's finally REALLY dead, not a hovering, damning presence, not a ghost alive in Yuriy, DEAD).
Adam and Yuriy at the end are themselves, without the ghost of mom, the weight of Abalkin, the curse of the shoes.
I didn't get the ending...frankly Adam enjoyed seeing Yuri get fucked by those old assholes and suddenly he did a 180 degree flip and is all territorial about yuri?? And the next thing he says is he loves yuri? It was too rushed and lacked justification.