Just before that, in the courtroom they bring up his childhood trauma of verbal, physical, and sexual abuse by his parents, and his entire mood shifts, scribbling manically in his notebook before a musical number goes off in his head concluding with him shooting himself in the head. And the guards are raping him in retaliation for outing their physical abuse of him in the courtroom. For the rest of the film he appears to be fighting suicidal ideation, and intentionally sheds the Joker persona publicly so that his miserable life can finally end.