Literally he's always been a yandere, its just it wasnt as bad until he got triggered by Yujin leaving him.
People tend to forget he couldnt understand or feel emotions, he learned to act them by studying other people as a kid, he was manipulative, possessive, ors just that he wasnt abusive, he has always been somewhat morally grey, he cannot be classified under the categories Red Green or black flags, he is a mix of all depending on the situation. They also forget he qas obsessed with Yujin ever since he had seeen that video of his so many years ago, he's been trying to find him since ages, and since he succeeded in that and assumed it would be permanent but got stood up, he lost it. He has always been obsessive, possessive and a bit crazy
What else is it supposed to be?
Yall cannot be mad at a morally grey character for acting morally grey.
Yall mistook his nature ro be as green when in reality it never was, it was toned down red flag at best. He was gentle, loving yes, but he was also manipulative and obsessive.
He's literally shown to have sociopathic tendencies omg
How are yall mad at the author for writing Aaron the way he already was!

all of u whining and crying as if aaron hasn’t been this way since the beginning yes he treated yujin well but he was never a green flag