
I know this is not smth you want to hear but I'm still gonna say it, Haesoo prioritized no one but himself/his emotions that time. But if he only step back a little he'll realize that no matter how much Taku begged him, the decision is still gonna come from him.
With that in mind, Taku will get hurt no matter what Haesoo's decision ,yeah, but ending it when 'it' never even started is the right thing to do. Haesoo was hurt that time? Now look, he hurt Taku and Joowon in the process. Do you think that's acceptable?
I mean, ok fair enough it might be pretty common in fiction but you normally do not use others like that, yeah? I sympathize w Haesoo but I cant make myself agree or accept his decisions.

"The decision is still gonna come from him"
You can say the exact same thing about Taku though? It takes two to tango after all
Honestly Taku was the more emotionally stable one out of the two of them. He's not the one going through a sad and failed relationship after all. So given those circumstances, I dont have much sympathy for Taku, since he made the choice to persistently advance towards Haesoo using his own judgement. Like there are a couple instances where Haesoo even tried to back away like, "yeahh this seems like a bad idea", but Taku literally showed ZERO hesitation? not once did he falter in his decisions unlike Haesoo who flipflopped with his bad decisions lol
Also not to mention the fact that considering your original comment... Joowon did his fair share in hurting Haesoo too. Not just the other way around. so I think you're just being biased. The way I see it, none of the 3 characters are "better" morally than the others.

Believe it or not, A hard 'no' at times like that is enough to stop it all. It wont be good for the plot progression but that's the most realistic decision imo.
I like Joowon yeah but this isnt about 'I want Haesoo to end up w Joowon' but more like I want Haesoo to be brave and face everything head on (preferably, right from the start but he didnt )- not unintentionally hurt himself and drag the others with him. Also, I dont think I'm being biased. I dont like any ships and I like Joowon as a character, I only said 'Joowon, be happy' as a sidenote, that's all.
Plus, about the two's painful relationship, Taku did this to himself and Haesoo should know better.
I honestly do not like Haesoo.
I hope this is it, the happy ending confession scene, come on now.
I know it was his personality, the lowkey kind, pretty low profile and discreet main character but gosh darn it IF only he just flat out said no to Taku and actually not entertain the thought of using another person as a test subject of some kind. If only. Then, he'll prolly may have a rocky relationship at first but at least he's with the one that he supposedly love since the dawn of time... Joowon, hun, pls be happy.