Lol I see the reason he's being called 'the worst' is because in a real korean workplace setting, his bluntness is considered pretty abrasive and will cause daily discomfort, but not far enough to be called as legitimately bad, just doesn't make a comfortable working atmosphere, and since you're usually in itfor the long run, it's just 😩.
Plus, he's a team leader, which supposedly also lead in keeping the team's morale & social connectedness up etc
With all that said, he does seem to be an effective team leader in terms of getting actual things done, just not a great one in the social aspect so the team members can't really legitimately complain, which can be annoying, thus the collective pent up annoyance inevitably created the reputation lol
I think also because we are too used to dramatic villainous characters in manhwas (where 'bad' characters villainy are amped way up), our standards for the 'worst' label is probably prettttty skewed lmao, while this webtoon setting is quite surprisingly grounded, as far as korean workplace setting goes.