How do we perceive good and evil?

What is the way, in which we perceive good and evil? Is perceiving good and evil selfishness? What is good, and what is evil? What is evil and good, is usually well comprehended, but why is not one comprehension of good and evil acceptable to all? Even in one family, family members comprehend good and evil differently, from one another, sometimes. Perhaps, all negative perception is bad, and all positive perception is good. I am never on the side of one, who justifies his negative perception, at the cost of someone else's situation. When one justifies negativity, then one is being perceived by someone else as being evil, or bad, and, negativity is being justified all around, then, and hence, one cannot say, who the particular guilty party is.
How exactly do we perceive evil, and good? When we perceive another person taking advantage of what they see is a negative situation, which pertains to us, to their advantage, then we perceive evil, on their part. Perhaps, the opposite of an evil perception, in my interpretation, is a good perception.