This is a terrible turn. Not even because of the story idea, or the characters' actions(the characters are acting BAD here, though), but because of who we're seeing it through and what we know. Last chapter we had a setup for a flashback arc, which was the natural way for the story to go; we don't know much about Washio's past, even though its important and the entire foundation of the manga.
The manga's story is a mid-life crisis. The point is that the main character feels like he gave up his shot in life, especially since he was summoned here as the legendary heroic hero. We didn't know much about how that all came about, just that it did. The pivotal moment seems to be around the monster girls, who were just regular-ass monsters when he saved them. The dude threw away his life for them, but they come back to remind him that he didn't waste his shot, he did something good.
So its a little weird for them to suddenly become a problem, and even weirder that they let him leave and believe him when he says what he said, and the weirdest that we're now following the MONSTER GIRLS on their quest for the truth. They know the truth. We don't. We need them to tell us what happened, not for them to find out.
Washio may have had every reason to walk out, we don't actually know much about the events that took place; but the monster girls sure as shit do. He saved them. That's why they're so attached to him. They know the facility they were in, they know what he did. It's even clear in that Washio is tailoring what he's saying; rather than saying he magically brainwashed them, he says "I tried to hide but you guys came and found me, if I knew you were going to do that I wouldn't have helped you." Rather than saying that anything about what he's accused of is true, he's instead saying that the only problem is that they came and found him.
There can, honestly, be a really compelling reason for Washio to be acting the way he is. He did, after all, basically give up on life. We see his depression apartment in the first chapter. He may have been laying low because he made very powerful enemies, and by saying what he did he'll keep scrutiny away from the monster girls and needs them to hate him so they don't follow and get into trouble. He can be facing the fate he's been running from here, or just frantically burning bridges behind him so that nobody can follow him where he goes. There CAN be a reason for this. Washio and the monster girls would know it, but we don't.
No matter what, though, this is a terrible turn for the series, unless this is the prelude to an axe; in which case it's a terrible turn because of the axe. There are so many more characters to see and so much more to learn about everyone and everything. Having something this dramatic and conclusive happen while we're still kind of in an exposition and build part of the story is premature. Everyone isn't in place yet, we don't know why Washio's doing this, we don't know who or what he's up against or what lies are being told about the past because we don't fuckin' know anything.
I honestly love the idea of this manga. It's pandering as hell with the big titty monster girls, but there's an emotionally resonant core here. An isekai'd guy who rebelled against the cruelty of the world he was summoned into, but fails and becomes a shmuck instead of anyone important has a real pathos to it. It's an engaging idea, and the things he threw his life away for coming to find him and convincing him that he isn't a failure, his life isn't over, and he has a lot to give is a particularly great setup for satisfying growth and exploration of the setting.
I honest to god don't understand how this arc is as big of a fumble as it is. I don't know how anyone thought it was a good idea, unless they were desperate to subvert expectations and shake things up. Especially since we haven't even introduced the full god damn CAST yet. We saw SO many more monsters in that classroom, that's SO many characters to look forward to, and we've seen MAYBE 3? Cmon man