Evil, like history, belongs to men alone. If a tiger escapes from a zoo and attacks you, it would hopefully not cross your mind as being an evil act; and even while being brutalized, you would know that the tiger does not hate you, or is attacking you from malice; it is just being a tiger.
So people act with purpose. That gives them a capacity for evil. And yet to know if their actions are evil or not, you must know what that purpose is. Its context. Their purpose may even be just, and yet their execution of that purpose is what has resulted in evil.
Evil may indeed be an act of commission or omission. So unless you actively oppose it, you are accessory to it.
Evil is also dependent on its recipient. An act of little consequence to an adult might be terribly oppressive to a child. And yet it is not fair to call an otherwise normal and neutral act evil, just because who it is directed at is weak, and feeble of spirit. Except when directed at them for just that reason.
It might be said that suffering is evil. Not causing others to suffer, but suffering yourself. If you refuse to suffer, despite injury, then you transcend being a victim. From that point on, even if the intent is evil, it is just reduced to a contest between their seeking to harm, and your seeking to protect. Win or loss, it is just a contest.
In the end, to understand this abstract of man, you must also use another abstract: judgment.
I think it depends on how you define the word. To me, evil is the act of diminishing life. Life is free will. Take away a man's freedom or his life and you are doing an evil thing.
But many living things depend on killing to survive. And we need to incarcerate criminals. So we have the concept of a "necessary evil". We kill a chicken so we have food but tell a child it is wrong to kill a chicken for fun.
The principle is that we do things that serve the greater good. The life of a chicken is less than that of humans. And the freedom of a criminal is less important than the lives of his many victims.
Moral relativists don't do calculations like this. They will say that Saddam did things things that were good looking at it from his perspective. But if you tally up the sum total of human misery and subtract it from whatever enjoyment was derived from the pleasure of Saddam and his minions, you come up with a big negative.
Except of course for the angelic domain and angelic conflict.
Returning to Plato's Euthyphro, Socrates advanced the argument that piety to the gods is impossible if the gods all want different things.
Morality is impossible, because all humans have different morals... Claims of morality is sophistry without some singular higher power defining what it is...
But, since we are all properly obeying the * modern interpretation * of the First Amendment, good & evil isn't the question... Good & bad, right & wrong, etc., etc., ad nausea; are all inherently religious ideals.
The modern interpretation of the First Amendment (according to the liberal-tarians) says government must exorcise all traces of religion and theism from itself. Therefore, government must never consider issues of morality and right and wrong.
So, it becomes a question of benefits versus costs. Fetus killing has its benefits to society, especially if you like to sleep late on Saturdays. But it also has its costs as well. Society (by which I mean, whoever manages to seize power) needs to evaluate these costs and decide accordingly.
The mythical rights of men and women are also meaningless. The very concept of rights is also founded in religion. Since the enlightened person is freed from any superstitions about some "God," they are free from having to worry about "rights."
Only raw power counts and humans are just meat puppets for the powerful...