Even though people may not be deliberately invoking religion when they talk about evil, that is in fact what they're doing. Behavior may be concretely described as unlawful, destructive, disruptive to social order, but evil? That's always going to be up to interpretation, and there is simply no single authority on Earth that can dictate what is evil and what is not. That's what brings it back to a religious argument--something greater than us must be the arbiter of those distinctions.
The example you gave with the ancient Greeks actually helps make my point: they considered disorder evil, but that doesn't mean it is. Even today, we recognize some amount of disorder as beneficial to society. Definitions change. Subjective labels like "evil" serve very little purpose when analyzing motivation or consequences.
Your closing phrase, "evil as a given behavior," doesn't even make sense to me, because I cannot describe any particular behavior as objectively "evil." I could describe it as unethical within a given ethical framework, immoral by the standards of a given society, physically or emotionally destructive or disruptive, but not "evil."
You're basically saying it's up to God, or only the theologians, to define evil? No other definition would be valid?
Of course the term is subjective. And, if the Greeks considered disorder evil, who am I to say it wasn't? It certainly seems logical to me that a society would fear chaos and the things that come from it. Evil things can't come from chaos and disorder? Chaos and disorder are often the results of natural disasters, which some Christians even today blame on Evil. So, why shouldn't a society call disorder evil and try to prevent it?
Who "causes" these evil things, who wrought them upon the Earth, is a question for theologians. Making evil empirical and trying to explain it within the empirical world is the job of science.
Why God would let Hitler come to power is not an empirical quesiton. There is Evil in the world is an answer. How a man could become Adolph Hitler, how a society could fall under his spell, and why he acted as he did are empirical questions. And, it is a study of evil. I don't think anyone would dispute that.
Many political and social terms are subjective. What exactly is sociopathic behavior? What is paranoia? What is partisanship? There can be valid degress of each. So, just because evil is a subjective term doesn't mean there aren't reliable and valid definitions of it. Or even recognized variations in degress of it. The word "evil" gets overused, politicized, and has been watered-down in popular use. But if one wanted to really bear down and conceptualize and operationalize a scientifically reliable and valid definition, one could. Indeed, one could create several. All good.
If we dismiss all definitions as merely subjective, then we're saying the subject can't be discussed at all. We have no common ground other than just agreeing that it exists in an abstract form. In religion, this may be fine because it's the existence of evil itself that's more important than any one empirical defintion.
Any evil, however defined, must be reconciled with the existence of God.
Pornography is hard to define. It is highly subjective. But we don't walk away from operationalizig it. We have legal defintions of it anyway. We dispute them, refine them as mores change, but we nonetheless have laws that are at least acceptable to a majority of those under the various jurisdictions which enforce them.
When I say, "Evil is a given behavior," I mean I can define the concept and operationalize it. That is the basis of any scientific method. It's not necessarily always about "why" evil exists. That is philosophical and theological.
Sometimes it's about explaining the manifestation of evil. It's employment, and how mankind got to that state. How what's evil even changes over time. Today, we would consider owning slaves evil. Yesterday, some people justified it by saying it was in the Bible.