I have thought about this topic alot and read also much about it by philosophers.
Without going too much into details, my own worldview today is that, if you consider that the aim of any life form is to maximize biological fitness, then in the long term "good" is simply intelligent behavior that sustains life and "evil" is unintelligent behavior that defeats the purpose of existence.
Indeed, no matter how you look at it, even if your goal is to be as powerful as possible (an apparently "evil" goal at first), you cannot diminish the health of people below you, or else your power diminishes also. It is in your interest to maximize the health of all people depending on you, so that you are also able to do more things thanks to their superior fitness. What good would it be to have an army at your command if your soldiers are not at their optimal level of health. You wouldn't want to hurt them and you wouldn't want to waste them! A commander that would not care about his soldiers, and would just hurt them for fun or waste them, would simply be stupid.
Seen this way, true "evil" would simply be stupid and impulsive behavior that would bring you to destroy your own health and that of others - which is illogical. "Evil" would just be another term for "lack of health" or "idiocy". "Evil" would simply be sloppy behavior and an excuse for a lack of conscientiousness in using power.
Now, if you look at it not in "long term", but in "short term", then there is a twist. Apparently "evil" behavior, such as being tough, pragmatic, uncompromising, disagreeable, can actually be "good" for the system in the long term. Apparently "good" behavior, such as giving, helping others, self-sacrifice, can be detrimental to the system and "evil" (Hell is paved with good intentions).
In sum, if you look at things tactically, you see that evil/good do not matter really, and you must be able to do both. If you look at things strategically, there really is only good, as steering away from a direction of strategic "good" is like steering away from life.