I know they’re probably gonna make up and the brother can finally rest easy and none of this will matter but still
I think the Mc said that it's not that he just remembered his past life, more like his past self fused with his current self, so all those terrible things he went through because of the king weren't just distant memories from a past life, but actions that still affect his current self, so maybe that affected his decision making. Also, maybe in a way, not being able to stay close to Mc was indeed the brother's punishment for his past life's actions, I do think he has a chance at redemption at the very least since he seems remorseful for what he did unlike the other guy.
I can understand his choice. I have a half brother that I grew up with that I no longer speak to and barely care to hear how he is. May sound harsh but when I was a child and he was a teen he tried to kill the neighbors child, he was going to drown him because he didn't like him. As a child I had to stop him from murdering that kid who did nothing wrong. That half brother did various other shitty things to me as well but that was what sealed the deal on not talking to him. So I can understand the MC not wanting anything to do with his brother, I'd do the same thing in his shoes.
I understand that what the brother has done when he was king caused a lot of grief so that would affect how MC treats him now but how can u basically act like he isn’t ur literal blood brother that u grew up w/ in this lifetime just like that?? I was just surprised by how cold MC was to him, already treating bro like a stranger when they grew up together under the same roof