IIRC Mercury Fred points to a "pattern" of deception and bad behaviour going back to 2016 that he considers disqualifying. Examples include delaying release of tax returns, not releasing "full" medical records, not doing enough to disown toxic supporters, refusing to drop out of the 2016 race, etc. I'm sure there's much more he'd refer to with no hint of irony. I think in some cases many who vehemently oppose Sanders supported Clinton in 2016 where he, of course, emerged as enemy #1 and remained that way for months. Anything he did went through a certain hyperbolic filter and it became impossible to dislodge for some. Online arguments with Sanders supporters no doubt made this far worse. Completely understandable, though it did mean when he launched his 2020 bid it was met with a built-in host of detractors who weren't about to back down. This isn't to say that anyone who dislikes Sanders has the same origin story, or that there aren't valid criticisms of him or his campaigns, but I find this is a pretty common byproduct with people who were invested in the Clinton campaign.