So, hot take: this villain's plan is logical.
Hunter guilds have more power than government bodies in a lot of these sorts of settings because the service they provide is so damn vital. Authorities will often look the other way on most shit short of active rebellion.
If he really is the only one who can produce antidote, then his position actually is pretty secure.
Our boy may be able to beat anyone in this world in a fight, but he's not omnipresent and the villain kinda has him by the balls here.
That all said, MC was literally stabbed in the back by the people he bled to save and he didn't so much as twitch an eye brow, but he is about to blow a couple gaskets now. Plus the "nothing in this world" line has strong "no man can kill me" energy.