
Yea but like how is it possible to fall in love with someone abusive uk. Like he might feel like he has a voice but does he really have a say in anything that happens in the story. Also he's justifying ilay's actions saying that ilay is doing this and that cuz he doesn't realise his feelings. I am an engineering student

Indeed its hard to justify it because I haven't finish reading the novel which will be hard for me to deny that its stockholm, but based on the manhwa, I could say that he doesn't really have it and it's hard to conclude if its really stockholm syndrome when we haven't seen what comes after the realization of Taeui that he's in love with Ilay.
Taeui still can fight back and defy Ilay as shown in numerous instances, and he doesn't show the submissive side of Stockholm syndrome, which is unable to defy the orders of the other person. Taeui still sees Ilay as a threat, which contradicts what Stockholm is. He also has that will sometimes to escape on Ilay's grasp.
Remember that Taeui is with Ilay to find his brother. If the fact that Ilay will help Taeui to find his brother is gone, then Taeui have no reason to be with him anymore, he survived alone without Ilay, and he doesn't have the longing of being with him.
Well, assuming it, the reason why Taeui sometimes gave up on escaping from Ilay is maybe because he has depression. His parents' passing and his brother leaving his life may have affected his whole personality, Tay sometimes shows a depressed mood or irritated mood, he can go to both state easily very often. although not mentioned, he also sometimes feel guilty on others death, he also feels guilty always about others life, like in the situation between Xinlu and him even though he did nothing wrong, he still feels guilty. He also shows low level energy and efficiency in everyday life.
Maybe the reason why he started having feelings for Ilay is because he has a longing for love which disappeared from his life and he doesn't know it. I mean knowing he sometimes have sexual intercouse with random people, he might be replacing the emotional love he receive from his brother physically to other people. And because Ilay started to show signs of falling in love, he started to accept him as a replacement from his brother.
Haha sorry for the unorganized explanation. This is based on my observation, only the future chapter will confirm whether it is right or not

I'm not in the field and I really don't know the science behind it but think it gets worst at the end of the novel (even tho probably inst Stockholm by the points you brought):
SPOILER A HEAD:
tae is taken a part from Ilay but it's planing to get back to him bc he realizes he loves (?) him, but at the moment Ilay sees him he capture Tae. He ties up and muzzle Tae, and start to rape him as a punishment, after A WHILE Tae is able to speak again and confess to Ilay, then they stay together
babe it's not schizophrenia, you are developing stockholm syndrme