I almost feel like they didn't know where to take his character at that point and he was dead before they had to write more for it. I feel there was still development to be there (at least enough room for him to realize that accepting Mondo's death would require him to get past this weird fusion that his brain had created in its grief state) but that's the liberty they get with this game, it would have been silly if everyone's plot points had been all wrapped up neatly before each of their deaths.
Another question, was I the only one that felt that Junko's split personality.....thing was a mock of each of the characters? For example, the high and mighty Junko with the king hat reminded me of Byakuya, the aloof Junko covering her face with her hand made me think of Kyoko, the cutesy Junko made me think of Hina, and obviously her holding Monokuma up was just her playing the Monokuma character again but it also reminded me of Genocide Jill (which is probably more due to the fact that Erin Fitzgerald did both of those voices).