“…We have all got both light and dark inside us. What matters is the power we choose to act on. That's who we really are."

I don’t see anyone as being distinctively good or evil. Leah Kleppinger should not be considered the “epitome of good” just for returning a wallet; nor is the individual that would have kept the wallet the archetype of evil. We cannot even go as far to say that Leah is generally an altruistic person. In this particular situation, Leah acted out of respect and benevolence. And we appraise this as so. But we tend to overlook the motives of any given response. We instinctively condemn those who steal, murder, or destroy. Ultimately, a wrongdoer with reason or purpose is more virtuous than the humanitarian with corrupt and immoral values.

The idea of something or someone altogether pure and good exists, as does the idea of evil and sin. But, I do not believe one such idea can exist on its own, much as certain atoms cannot exist in the natural world as a single entity. They must bond—they are always part of a whole. Humans are comprised of both good and evil, just the same. And this is where this ongoing battle rages between the polar ends of this two-way spectrum of human nature. As Sirius Black so duly states, “You’re not a bad person. You’re a very good person, who bad things have happened to… Besides, the world isn’t split into good people and Death Eaters. We have all got both light and dark inside us. What matters is the power we choose to act on. That’s who we really are.”

Good and evil… Selfishness and selflessness… Ethical and unethical… In some respect, they’re all the same. And there has never been a time in human existence where these forces have not been at work. Honestly, I am more inclined to believe that “evil” hold a greater power over all of us. We must make choices between what is right and what is easy, faster, or more advantageous. The temptation to regress into our wicked, egocentric selves is something we must always (with great effort) confront and triumph over.
The final question of whether evil can ever be eternally eradicated can be answered by the simple statement from Albus Dumbledore:

“It is important to fight, and fight again, and keep fighting, for only then can evil be kept at bay, though never quite eradicated."