Responses
His reaction, in my opinion, is understandable. At the begining, JiWook's experience with Seungeon wasn't great. Seungeon treated him awfully and than there came Hyungseo with his fake goodnes(which of course JiWook doesn't know is all an act) and treats him nicely. Of course he wouldn't want to believe what a guy who treated him rudely when they met said about guy who was kind to him. Just like you said, some things we as readers know isn't what characters themselves know about yet and I feel people forget that sometimes and thats why they get annoyed with some characters actions.
Honestly at first when Jiwook didnt believe Seungon about blondie being dangerous, I was frustrated, like how can you be so dumb and stuff. But then I realized of course he would be skeptical because if I was in that position, in that toxic school , and only one person was nice to me, I wouldn't want to believe such a thing about them either. As readers, we know blondie is super psycho because of that teeth pulling scene and hearing his inner thoughts but if we saw him from Min Jiwooks perspective, he wouldn't seem so bad, maybe just slightly sus. So its no wonder he doesn't want to believe Seungon immediately. Is he naive? Yes. But I do think it's a sign of good character that he judges people by what he sees rather than going by rumours. Although in this case his judgment is really wrong so hopefully he does stay away from blondie