There are many aspects of good and evil. But most fundamental might be:
Given that existence exists:
Is that good, is that bad?
Good people think it's good. They follow the path of Life.
Evil people think that reality as such is so intolerable that it deserves to end. They will destroy even their own conception of the truth in order to bring about the cesation of their own temporary suffering, or to bring about the cesation of them having to behold an existence that doesn't meet their standards. They will offload this suffering and this prideful vision of how the world should be shaped to others, and they will work toward the destruction of what exists. They remain devoted to the idol of their own arrogant will to control aspects of reality that they have neither the right nor the power, ultimately, to command. Because of the folly of this endeavor which can be demonstrated by universal laws exhibited by all of existence, laws to which they are themselves subordinate, the existence which does not meet their prideful standards gradually slips through their fingers, and the life is drained from them until they meet the demise they so crave, if only for themselves. In short, they experience the Tarkin effect until the Death Star blows up in their faces.