@Mojo Beating the shit out of them will just cause the townspeople and Carol to turn against him. That will make the situation even more sticky, and considering Carol is an amazing healer, and a precious friend to him, Gran's party will use her, etc. Long story short, things will get really messy.
And even if he does tell everyone that they're brainwashed, they won't believe him because they're brainwashed, or will be brainwashed to not believe him later. They might be even locked away or left to the monsters.
Also, the backlash of hurting the user badly, or killing him if you want, is rusty like someone said in the comment section. What's worse, if he's using an item to brainwash them, and what happens if it gets destroyed.
One wrong move against something that you don't know of, might just be game over. Doing anything rash isn't the best option. Kaido needs to figure out first how the scum's doing that一 using magic, an item, or what一, collect evidence, and plan from there. It'll also be nice if the capital gets involved.