This world is full of them. According to our own personal standards, we can determine whether someone or something is good. The truth is that there are not absolute truths; good and evil are the two sides of a spherical coin. They are labels used to express personal criteria of satisfaction produced by environmental outcomes.
All right, then. If good and evil are as versatile as I want to take them, let me do what's best for myself, no matter how evil it may result for everyone else. For some, that's the way things should work, but not for everybody. In the absence of absolute truths, this world ends up being ruled by absolute agreements (as relative as personal long-term decision-making processes), also called conventions. Good is what's best for most of the people we interact with, evil is what's worst for them.
Good, better, best, bad, worse, worst, and evil are just ideals. The main property about an ideal is that it can bear no direct relationship with the real world—the one made possible through social interaction. Thus, social interaction ideals are unachievable, because the more worlds interacting—any living being, in the most pragmatic sense, is a world on its own—at any given time, the more noise needs to be added to the equation, and the more distant and ideal the actual result becomes.
When something is good, it can always be better, that's the core of the philosophical idea of progress. There's nothing in this world that can't be improved, there's no finish line. That's why things never get better enough: once they improve, a new improvement can be done, and, therefore, has to be done, and so forth.
The mistake, human's unavoidable error, lays on their ambition for absolute, pure knowledge and perfection. There is no such thing. There are no pure evil humans, and there are no pure good humans either. Except for the choice capability, there's no inherence in human behavior. There are only different ways to learn how to suffer and how to enjoy, and how to make others suffer and how to make them enjoy. No matter what your intentions are, society will judge you for the goodness or the evilness of your results.
Good and evil, this world is full of them. Some think that we live in the best of all possible worlds, and some others believe that the world we live in is completely improvable; I think that we live in the only possible world: a random, chaotic palace—improvable and arrangeable, nevertheless. But no, it won't get any better than this—not better enough, at least.