The term "hero" is just from a matter of perspective. If zero was on their side, he would've been a hero to them. Of course he's not going to be a hero in the minds of those he fights against. A "hero" mass murdering enemies isn't anything out of the ordinary but one of the most common ideas of what a hero is in history, one could even think of it as the norm. It's just recently where mass media and things like shounen manga/anime/novels have led people to associate the term with selfless saints sacrificing themselves for "the greater good" and "never killing anyone". Hell, even saints might be unrepenting mass murderers in history due to their cause but it's often used to portray someone truly selfless and kind. Many also like to call soldiers heroes even if what they've done is the stuff of nightmares to some others.
That aggressors would view zero as nothing but a murderer isn't abnormal, to them he's just a murderhobo, but that zero would've viewed his own actions as not being worthy of being called a hero is pretty stupid to me. Then again, I never liked his idea of justice in the first place. He has let far too many dangerous people live to cause mass destruction and death another day. He's like a kid with childish ideals on a power trip.