You said it : because he fucked up the mission, which leaded to Taeman being hurt and the love between Taeman and Daehyok being put in danger
Rather than hating him, I think people r just extra pissed at him rn for being upset n yelling at taemin, since technically it was his fault for being overconfident yet not being able to kill pinkie pie
I actually like him quite a bit but the current situation was really caused by him being stubborn
Personally I hate him because he already HUGELY fucked up and still acted high and mighty after. If that were me I wouldn't even be able to look at them in the eye. This guy acted without his boss's consent. They already told him he WASN'T capable enough to take on such missions yet and this particular mission was complex. And what does he do? He acts against the wishes of the person he supposedly 'idolizes' and fucks up an important mission almost killing Taeman and compromising their entire organization as a whole. He's not doing him a 'favor', what he doing is barely enough to cover all the damage he's done. It should be a given that after fucking up so badly that he should do everything he can to make up to them. He should be thankful they haven't even discarded him yet and decided to keep him because Taeman's cover is now quite literally blown to ashes and this once capable guy can't even perform missions anymore because he's on the run not because of his own mistakes but because a rotten banana looking mf couldn't control his own fragile ego. I'll also correct you on something, I'll still call the hell out of someone's bs even if they're in the main cast and I can love side characters just fine even if they're not the MC or ML thank you very much.
ATE EVEN THE PLATE
Idk why is there all that hate on blondie like he's doing taeman a favor that's why he's there and before when he fucked up the mission he just wanted to prove himself to the person he likes like y'all really hate complex characters when they're not the ml or the mc