How do you define "hero"?
I haven't kept up with him, but from what I remember, Venom started as a villain due to his vendetta against Spiderman, but kind of evolved into a "vigilante", sort of like Punisher, as he fought against Carnage and the killing of "innocents". I don't think he goes and fights other supervillains just because they're bad though, but if "innocents" are involved, he'll get involved.
So I don't think such individuals are "heros" in the truest sense of the word, but they're more "good" than "bad".