His reaction is half valid, he shouldn't have pounced on him with allegations and assumptions + insults that could halt Jintae's progress in change without looking at the outside picture yet aka the fact that Jintae is in DAWUN'S house and speaking to him as he thought it was Dawun at the door in a comfortable manner like they live together which they do but he wouldn't know that. He could've questioned him all he wanted and developed his opinion then + heard him out but instead he went full coo coo pounce on the guy. Jintae has apologized for all the he has done to them and caused for them enough and I'm not saying they should just forgive him and let things go but they said they were willing to move forward so this should've been directed in a better route no matter how high his feelings were.
Again I agree he’s immature for jumping without thinking- I js don’t think his reaction is that insane. Idk why you find it so crazy he’s cares abt dawun business. Looking at it they have a sibling type bond (you could say). My sibling literally butts into all my business that pretty normal thing ( and Me and her aren’t even that close).

Why you guys being so hard on the seonghan bruh. He’s literally clueless abt the situation. To him the person who broke his bf heart is randomly in the house of the person he looks up to for everything. Not to mention he thinks dawun didn’t wanna be marked with a scent. Which is obviously bad thing if done without consent (ik dawun didn’t see it that way). He also hasn’t been seeing that jintae has been trying to change. He was immature but He’s justified in his reaction based on the information.