I also forgot a lot of details but a few I remember being he used to get punished by his family but wouldnt react, his family was worried, so he learned how to react to pain by watching actors on a TV, and then acted it out infront of his parents, which suggest he has sociopathic behaviour somewhere deep down.
He also "jokingly" pointed a gun at yujin maybe in the beginning somewhere.
He got jealous and told Yujin "to take it like a good little pet".
Not to mention he was beyond obsessed with him ever since he saw the music video Yujin had uploaded, he literally searched/stalked him everywhere to find the "guy", He went through Yujin's files and used Yujin's situation to make a deal that he could take advantage of, aka making Yujin write the song for him that helps him sleep and have sex with him.
He literally put a tracker on yujin's phone, clothes and things so he could track him incase Yujin runs away.
He also spent so many fucking dollars just to find Yujin.
He also pretended to cry infront of that bartender to get information on where Yujin resides, once she told him out of sympathy, his eyes turned piercings cold, indicating he was manipulating her and was successful at it. He also threatened her that if she helped Yujin again, He would make sure her son wouldnt be safe aint no motherfucking green flag does all this.
Not to mention he has severe insomnia, unless when he listens to songs made by Yujin or if he is with yujin himself, which shows the obsessive, possessive nature runs deeper than just physical being, its emotional and mental too.
In this situation, he practically cannot breath without Yujin being near him.
I mean I call him red as it's a bit easier when I'm thinking on his direct behaviour but he sits very much in the grey.
In that sense it's a bit like hating on a lion when it chews your leg off. Just because it learned not to bite you in the confines of a cage doesn't mean it wouldn't bite once free.
In the "relationship " Aaron controlled himself to a degree but once he was free of that he reverted to his true nature.
I'm very interested to see where the author takes this.
I'm also very tired and am waffling but I hope you get what I'm saying in my long winded, adhd way!!

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,its 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.yall also forget he was obsessed with Yujin ever since he had seen 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.
Yall cannot be mad at a morally grey character for being morally grey be fucking for real right now.