I think that we
humans are
fundamentally good, it's too
depressing otherwise.
People can tell the difference between good and bad, it's quite easy. Putting your hand in a fire causes damage to your hand, it also causes pain. We humans don't like pain, it is bad, that's instinct. We pull away, get our hand out of the fire, away from the source of pain. There really is no discussing if the nature of burning your hand is good or evil, that's just stupid.
This can be expanded to whether the whole of human nature is good or evil, I think we know that some things, some actions are evil, and that generally human kind shies away from doing evil acts. It embraces goodness, it strives for happiness, and at our cores, the truth of it is, that there is good in all of us.
It isn't necessary for some people to do evil acts so that we might recognise evil for what it is, and thus know what difference between it and goodness. There doesn't have to be evil for there to be good, evil doesn't have to exist at all.