Hillary losing broke him. He's more or less well-meaning and took a bunch of the criticism towards him seriously and attempted to learn.

https://newrepublic.com/article/155866/happened-peter-daou

It's kinda like how Ta-Nehisi Coates' turn on Israel-Palestine happened because of Rania Khalek yelling at him and others on a panel about reparations a decade ago.

Getting repeatedly beaten down by being a Sorkin style democrat eventually caused him to re-consider the entire American political system, iirc.

"Why should the best case scenario be harm reduction rather than not doing harm in the first place?"

"Why would a "democracy" that exports "democracy", often violently, not behave like a democracy with lifelong judges, electoral college and gerrymandering?"

"Why would the establishment fight Bernie and progressives with more vigor and ferocity than Republicans, who they're openly willing to compromise with?"

He started evaluating questions like that

From what I remember it was the abolish ICE movement getting immediately brutally crushed by Democrats that seems to have been the clearest catalyst. Up until that moment he was buddy buddy with the establishment and apparently fervently believed that they were essentially good, and their open pivot to "no, actually we like the ethnic cleansing machine stop saying it should be dissolved" cracked that and the way they turned on him for opposing them on it did the rest.