Seongmin is indeed bad. Dohun may have some grudge and wrath towards him, but he is a reliable narrator, mainly because his emotions dont interfere with how he sees people- for him, it's either just blank people or people who are involved with Yuwon. He can't see anyone's face other than Yuwon, but he can sense their emotion since he was taught this way from childhood- fear, envy, greed- he is excellent at sensing and distinguishing them. Of course, we wouldn't definitely know -since all three seasons were from their pov and the 1st season was from 3rd person's pov, which didn't know Seongmin well and mainly focused on Yuwon. Still, dohun's pov is the closest to the truth- because he has no reason to hide the truth; he has no reason to badmouth someone when he can just kill him off without a single thought.
Is seongmin bad? Idk I don’t think Dohun is a realiable narrator. I think he doesn’t like that has a place in the MC’s heart. So far the baddest thing we’ve seen is seongmin not defending the MC when the coworkers were badmouthing him…which again may or may not have actually happened…