Well, you can't say he didn't have any reason to change like that. Jaeha broke him. He should pay for what he did. This new Eunho is the result of his own actions
I don't think that's how it works. If you used to love someone, like Eunho loved Jaeha, you don't want to hurt them back. Now Eunho acts complitly immature and selfish. This " you hurt me so i will hurt you" act is absolutely pathetic.(sorry Eunho fans but i always despised this behaviour).
And in my opinion their relationship wasn't that good in the first place.
They adored each other of course, but as i see it this whole 'affair' was bleeding from several wounds. First Eunho's feelings. He may have been crazy in love, i think it was more like an obsession. I can understand that, he is pure af and he had a huge crush on Jaeha since he was a child. But in my eyes he only loved this 'idolized Jaeha'. Lets be honest, he is a sad and broken man who is afraid of the pain that a human relationship can cause, and for him they never ended well. He is so afraid that he would push them away before they toss him out, because not being needed hurts him more than being alone. And this is the real Jaeha, and Eunho didn't achieve to see it, and thats why it didnt work out for them. (Feel free to correct me if you think im wrong, i've read the manhwa a loong time ago, i may have missed/forgot something.) ╮(￣▽￣)╭
eunho is depressing the hell out of me, he used to be such a ball of sunshine and now he's this evil wreck of lust and self hatred and in the end he's just going to crash..jaeha needs to reach inside his heart and pull eunho out of this surreal arrogance he suddenly has because i am dEPRESSED