im gonna offer a crack theory, the crazy dude was actually trying to find the witch's brother, because he believes he's the monster cultivator coz he makes monsters too, its just that the nii-san monsters are actually dangerous and thats why the crazy dude has been so hellbent on killing every monster, and the brother was planning to make MC take the fall for the shit he has done so far
Disclaimer: i have not read any LN, WN or the raws of future chapters, im just taking a guess, thks