Heroes aren't always good people. Their reasons are lost to time to the average person, but the message is remembered. The actions supersede the reason.
Guy Fawkes is not remembered as a person, he's remembered as a figure.
every folk hero, just as, for instance, any of the national myths, are mostly entirely made up, and as more time passes, the less historical truth remains. This, simply speaking, is folklore, that has nothing to do with actual history, its more about communicating ideas and ideologies, that you imply with the story you're making up, tying it up to real historical characters is just a way of cheap advertisement. I believe, we must not ever forget that, as we are destined to have lots of useless fights otherwise.
Between Anonymous, Occupy, and proud boys all using the same mask, the only shared symbolism for the Guy Fawkes mask is really "resistance" of some kind. Which is so broad, it's hard to call it a symbol at all.
Look up Guy Fawkes. I don't remember all the details but I think he was some French guy who got executed for something.
Unless you're talking about that specific person wearing that outfit in which case he's from the movie V for Vendetta with Natalie Portman and Hugo Weaving.