I don't think they're accusing him of being the monkey; just that he's responsible for Tsuji's death in c9, who had his head bashed in with a rock, instead of shot by an arrow like most of the other kills confirmed by the monkey.
It's already established that Himuro's not the monkey (or there's a really elaborate ruse going on involving multiple people or robots or something) since at the end of c9, he's visible at the back of the group while everybody's facing off with the monkey.
Though, why on earth he's opening the window to make it seem like the monkey sneaked in is beyond me right now; maybe he's planning to kill somebody else to try and hide evidence?