Dunno, i consider fucking your best friends fiance regularly behind his back, never coming clean about it and letting him marry the girl whos cheating on him all the time without mentioning anything preeeeeeettty bad
While it doesn't excuse his abhorrent behavior, they weren't engaged at that point. Still, where the MC changed his behavior and (as far as we know) didn't cheat on anyone himself, I choose to see it as him learning from his mistakes.
Its kinda willingly dooming Zeke to a shitty relationship no matter what due to a reason the mc created in the first place
Zeke is his own person, who made his own choices and mistakes. Yeah, what Keyla and MC did was bad, but both him proposing marriage out of a fear of losing her, and Keyla accepting the proposal out of fear of being alone, play a much larger role in his current unhappiness.
Some may argue that Zeke maybe cheated too but in this case it would just have been one more reason to tell both of them to NOT MARRY if they are really supposed to be his friends lol
I don't remember if it's outright stated that Zeke had his own extracurricular activities. He does cheat on Keyla in the present, as a result of his dead bedroom/marriage.
Sure, Keyla shouldn't have accepted the proposal... But I'm guessing that in the MC's mind, he did the right thing by breaking things off with Keyla, and didn't see any value in hurting Zeke even more with the truth. Though, I do agree that from a certain perspective, the MC is a special kind of asshole who's able to sleep with his best friend's partner while keeping that friendship up.
I have no idea how this would feel in real life, but I imagine that the MC would have to be looking down on Zeke in some ways. Or maybe he's just into netori.