They wouldn't say that if they're reading this with their eyes open. Shin's not acting like that because he found out the truth about jay. He's like that because he felt 'betrayed'. This isn't about facts, it's about what he's feeling. You can't throw some 'facts' and 'rational' words to someone who's 'hurting' because of you. You don't use your brain to someone who's using their heart at the moment. You won't understand each other that way. You have to know where shin is coming from to understand him. You don't come up with 'smart' ideas using your head.
(I'm not angry at u huhu I might come off too strong)
Tho I would also do the same if I were jay. I gotta say that I would never even trust anyone no matter how much I love them, they could reveal anything to anyone once their life is on the line (but shin didn't do it even when there's a gun pointing at him and Jay witnesses it, it's so good damn) or to protect them, the less they know, the better. I think jay also felt bad for lying even if the reason is to protect shin. He literally let him shove the d inside his mouth to the point that the corner of his mouth is bleeding while letting him degrade him. Like damn, this man knows to love, might as well let shin fck him (kidding) but do you think it'll be possible? I mean, I think it would be very interesting scen—i should stop talking

Please, I just don't wanna see someone saying "he shouldn't even be mad, he should be grateful that Jay's not a thug" please get out