Maybe I'm too opptimistic, but as there are many temptations that drive people to do evil (asocial) deeds (mostly deeds driven be egoism in one form or another), there is also a strong desire/instinctive calling to do good (social) deeds. You might not give money to charity, but then again you might call an ambulance, if you see somebody bleeding on the street, who isn't responding. Nobody is pure evil nor good (though some might be really sinister), but for society to work, egoism isn't enough. Being able to trust each other and in turn get back just enough to feel your needs be secured for now, is soooo much easier, then picking a fight with the world. It's the lazy and frankly better way (most of the time, as most of the time it works out swell). So "good people" will always be in the majority, as it is build into our gnome and a proved concept.
Of course it works better, when evil people (evil proven by deeds) get purged from society. (Am I now in China-kun's army? Don't wanna.)
EDIT: Disclaimer: I'm not in favor of social purging. Just making sure.