
I see a lot of comments hating on Haero but i really can't hate him. He has his insecurities, he had witnessed so many traumatic things at such a young age and having no one to rely on has made him bottle up all of that and make him so fragile and insecure. His fear of losing Don to a girl in the future is a very real fear considering Don wasn't gay in the first place, his fear of being abandoned and his self-destructive thoughts are all valid.
And Don lying to him about meeting his ex-girlfriend really took a toll on him since he was so broken already. And his only chance at redeeming and forgiving himself died and Don wasn't there to help him get through it feeding into his self-loathing and fear. I'm not saying this excuses the way he treated Don, but it really explains it.

And from Haero's POV, he was doing Don a favour by leaving him. That means Don can have a normal life without prejudice or bias from society and he didn't want to be a burden to Don either (Don was clearly not mature enough to handle Haero's emotional baggages yet) Haero clearly love him enough to give up the idea of 'them' for Don's future.
It was wrong of him to just decide everything on his own and disappear without consulting Don but he was clearly hurting as well. He isn't a saint but he's not some devil out to break people's heart either, I hope they communicate properly and fix each other because this is too painful. ㅠ ㅠ
Also, Nami deserves someone who would love her to the moon and back
the old dude gave me actual goosebumps, the bad kind, the kind that makes you wanna puke, ew