"Elizabeth Warren, Hillary Clinton and New York Times columnist Bret Stephens have all helped perpetuate the idea that Sanders' supporters are somehow uniquely cruel, despite Sanders' platform and policy proposal being the most humane of all the candidates"

From Salon.com, There is hard data that shows "Bernie Bros" are a myth:

People responding to hundreds of millions of people online tend to dehumanize others.

They remember that someone is female/male or follows some candidate or is of some race, but they frequently don't pay attention to differentiate actions of one member of that group versus another. So rather than consider how frequently an individual of some group acts, they think of how frequently the group acts as a whole. If they interact with many more members of one group than another, that perception of the group is magnified by the number of members they see.

"Bernie followers act pretty much the same on Twitter as any other follower," Winchell says of his results. "There is one key difference that Twitter users and media don't seem to be aware of.... Bernie has a lot more Twitter followers than Twitter followers of other Democrat's campaigns," he added, noting that this may be partly what helps perpetuate the myth.

I downloaded all the followers of the Twitter accounts of the nine most popular Democratic presidential candidates and the president ([around] 100 million Twitter accounts). I then randomly chose followers from them and downloaded all their tweets from 2015 to the present. I have run two different sentiment analysis algorithms on these tweets. So far, nearly 6.8 million tweets from 280,000 Twitter accounts have been analyzed out of the 100 million-plus tweets I currently have downloaded (I continue downloading more).

The chance that some tweet is negative when it comes from a follower of candidate X is pretty much the same as if it came from a follower of candidate Y. [...] Bernie followers act pretty much the same on Twitter as any other follower. There is one key difference that Twitter users and media don't seem to be aware of. Bernie has a lot more Twitter followers than Twitter followers of other Democrat's campaigns.

To summarize my conclusions: First, there is a general tendency for online behavior to be negative, known as the online disinhibition effect — but it affects all people equally, not merely Sanders' supporters. Second, pundits systematically ignore when other candidates' supporters are mean online, perhaps because of the aforementioned established stereotype; in this sense, the Bernie Bro is not dissimilar from other political canards like the "welfare queen." Third, Twitter is not a representative sample size of the population, and is so prone to harboring propaganda outfits and bots such that it is not a reliable way of gauging public opinion.

"The idea that Sanders' supporters are somehow uniquely cruel, despite Sanders' platform and policy proposal being the most humane of all the candidates"

...is a pretty important phrase to reflect on!