I know this comes down to semantics, but I'm gonna explain how I interpreted it, both from the unofficial and official translations:
He knew his brother was in love with him, but I neither believed he was "obsessed" with him nor that he "egged him on." The egging him on part was implied by the other guy, but corrected by him: "Don't get the wrong idea[...]" (official)/"It'll be tough if you misunderstand[...]" (unofficial).
Coupled with that, I wouldn't say he was "glad" when he finally snapped, but rather that it was more an inevitable situation given the restraint his brother had practiced up to that point: "[...] I just thought the day had finally come" (unofficial).
With all that being said, whether it was what he wanted or not, he knew that his brother had so much potential and didn't want to drag him down to the same depths as him. It was only when he realized, even after doing everything in his power to give his brother a fighting chance, that kid was never NOT gonna be fucked up. I believe that was the moment when he let go and finally accepted that they could be fucked up together.
Well no I do get it, I just don't think it makes a lot of sense
If Motoharu was obsessed with his brother too, why run away? He initially said it was because he thought exposing his brother to the prostitution had negatively influenced him, but then the plot twist reveals he was aware of his brother's love this whole time and basically egged him on. He was even glad when his brother finally snapped. So why run away? He got what he wanted