You're not wrong that Sanders had some of the most toxic supporters. And still seemingly does. But there's a lot of reasons for that. Part of it was that many of them just like the concept of him and were not supporters of him for any particular political stances. He was the candidate that the GOP focused on promoting the most, to divide and conquer left-leaning voters turning them against each other. Something Sanders himself didn't want. But which worked in spades.
Sanders the person I think is genuine and a great candidate. I would easily vote for him AOC or Warren on any given day.
Third party votes in a presidential election are effectively virtue signaling. And nothing more. One of the many things that it signals is that you didn't really care who wins. Whether they are Fascist or not. Or that you were hyperbolic / disingenuous in calling them both fascist. Which don't get me wrong Democratic Leadership really seems far too comfortable with the fascists. But they certainly aren't the same.
It is still your god-given right as a voter to use your vote in that way. Though people are right to point out that you didn't show any sort of solidarity against the fascists. The Republicans margin of victory was so thin. This was literally the hypothetical everyone talked to you all about without end. Where you could have made the difference. But chose not to.