It's perfectly obvious that every candidate for every political office in the visible universe will find support from some statistically inevitable fraction of monsters, and that some of them will make their way online. Bernie Sanders has around 1.25m followers on Twitter alone. If general trends held, that would mean around 50,000 of them have antisocial personality disorder.
"But," you ask, "wouldn't Sanders be less likely to attract sociopaths, since his political movement is so admirably progressive?" The real answer is no, that is not how sociopathy works, but let's entertain the idea. How much less likely? Half as likely? A third as likely? We can even say that a Sanders supporter is ten times more likely to be sane and well-adjusted than your average person, and we're still looking at an army of 5,000 demented trolls who are feeling the Bern.
This point about sociopaths is just a way of putting into quantifiable perspective a broader issue that afflicts every political campaign. People have diverse and often deeply idiosyncratic reasons for supporting particular candidates, and they bring with their support diverse and often deeply idiosyncratic problems. Even if you are not an actual sociopath, it's still entirely possible for you to be a bigot of some sort, or petty and immature, or prone to losing your temper, or to have all kinds of other issues - for reasons that have zero to do with the campaign you support. When we are talking about populations of online supporters counted in the millions, it would be a miracle if a given candidate wasn't burdened with a substantial number of partisans who have serious and embarrassing problems.
So of course one should call out the racists, the sexists, and the belligerents who make actual threats. But if you want to take an extra step and point out that these people also happen to support Bernie Sanders - that they are Berniebros, as the slur goes - you have a basic question to answer: why?
If your argument is that Sanders supporters are uniquely terrible, it's obviously not enough to make the utterly trivial point that Berniebros exist, or to supply some anecdotes about some particularly nasty ones you ran into. All of that is easily and more simply explained by saying "yeah, there are some bad people out there"; none of it implies anything unusual or noteworthy about Sanders.
The more popular argument is that Sanders supporters are uniquely worthy of scrutiny, since the better political movement should be held to a higher standard. Setting aside the moral and tactical wisdom of this logic, it's enough to say that this argument is only plausible coming from people who actually support Bernie Sanders. If you insist that Clinton is actually leading the more progressive movement, or that she is more likely to win, then by the "higher standard" logic any extra scrutiny should obviously fall on her and her supporters.
It's of course perfectly obvious why a Clintonite would make a point of drawing attention to the bigots and belligerents who happen to support their political opponent. What we should not do is confuse that scam with a distinterested effort to call out problematic behavior for the sake of progress.