In defense of Joshu's actions in this chapter, they may seem wild, dumb, annoying, and disappointing to us (i too feel disappointed that he couldn't get a better arc too). But for Joshu's character and situation, these actions make perfect sense and should be understandable.
1. Joshu's father and the person he respects most, Joubin, have just died. His mentality was already bad. He was even under the guilt of not having saved his father.
2. He worshiped Yasuho like a goddess, of course he would want to sacrifice a part of him to save her, it would be a dream for him to have such a connection with the lady he loves. He was blinded by it, so he had the courage to take the equivalent exchange lightly at first.
However, he didn't really know what would happen to him during the exchange. So the pain could easily overwhelm his mind.
3. Next, he had a lot of pain, cracks on his face, lost an arm, in a completely unstable mental state. And Yasuho, whom he sacrificed a lot to save, didn't even thank him, or at least show a bit gratitute. And the literal next thing she did, was to ask to take the new Rokakaka (something extremely important) to JOSUKE-literally the man who he despises most, the man who stole his father's respect, his room, and "his" Yasuho.
=> Everything just kept adding up on Joshu's head and "his "loved one" giving the NEW ROKAKAKA to JOSUKE" was the last straw.
I doubt anyone can keep a conscious mind after all those horrible things.