The argument put forth isn't that there's a spectrum of good and evil, it's that good and evil don't exist. Presumably along the naturalist lines that we're all just animals doing animal things and morality is a useless fictional construct. An idea I find rather naive.

You lay it out so nicely and then rebut with "nah"?

I'd argue that it's a form of nihilism, and that nihilism itself is at best benign and at worst destructive. You can't build a functioning society or legal code based on "good and evil don't exist, we're all just animals". Even acknowledging that partial truth you eventually have to choose a subjective value system to make moral decisions. Perhaps it's a somewhat useful perspective in evolutionary science/research, but that's as far as it goes. I wouldn't recommend it as a life philosophy unless you just want to be an ascetic hermit, and there's a reason ascetic hermits are viewed as more zoo animals than anyone of importance, and they aren't very common or influential. Not that a true ascetic would care, their entire existence is eating their own tails to feel superior.

> You can't build a functioning society or legal code based on "good and evil don't exist, we're all just animals"

Of course you can. As long as the animals we are have the ability to understand the consequences of their actions, to themselves and others, and make rational choices about that.

I wouldn't put in prison or on death row a person who killed even a hundred people but functionally has an IQ of 45. That person likely couldn't make rational choices and couldn't understand what they were doing, why they were doing it, or what would happen next.

Whereas I would incarcerate a person with a 100 IQ who drove drunk in a well populated city, whether or not they hit someone.

Good and evil don't have much to do with either case.

What is the measure of an idea though?

The measure of ideas are whatever people can be convinced they are.

The trick is to make it sound like it's not you that's saying it. We launder it through God, society, reality, and it comes out sparkling clean to everyone who's gullible enough to believe it. It's a trick, but that doesn't mean it doesn't work.

Good and evil are products of the interpretive mind, one that absolve itself of its own interpretation which leads to the concepts appearing to stand on their own. They're social facts masquerading as common sense, "the way the world works", and reality.

That's not to say there aren't consequences to rejecting them as such, we have a whole system for social fact enforcement that includes social, physical, and mental punishment, but that doesn't mean that one can't appreciate them as social facts either.

Would evil exist in the natural world absent humans? If not, then evil is a property contingent on humans in one way or another.