One bad rung on a ladder will tell you that it's a bad ladder. Only checking every single one will tell you that it's a good ladder.
This is not quite true for a hero. There is the potential for spectacularly virtuous deeds that will establish the hero as heroic, but it's harder to do it than villainy, because most deeds are normally at least somewhat virtuous; no society could conceivably survive unless most deeds were. (This can be a flaw in world-building, where you have thriving Thieves' Guild in towns that could not conceivably produce enough to support all those thieves and the law-abiding -- aka working -- citizens.) Throwing a coin to a beggar might be a villain in a good mood, or trying to throw off attention, or merely a bureaucrat doing his good deed for the day and so absolving his soul of any need to behave decently toward anyone else; stealing the beggar's coin says "Villain!"
For that matter, many acts that would mark him out as heroic are too spectacular for the opening; we have to, after all, have some way to ratchet up the tension. Having the hero save someone, putting himself in great danger, or nobly refuse the villain's nefarious offer despite the damage he will suffer -- well, unless the story is one of unrelenting adventure, it's kinda hard to crack up the volume, and sometimes when it is.
So the hero usually needs a slow build-up of quotidian goodness to be the hero while the villain can be more easily delineated.
(Off on a tangent -- and then you get people who complain that female characters are either good-and-passive or evil-and-active, and they go on to define, say, saving the hero's life is passive because it's in conformity to social approval, whereas setting out to kill him is active because it defies social approval -- and so it's not hard to find only passive heroines and active villainesses.)