Orrrr it would mean that there's no such thing as moral values, as well as no objectively correct way of doing things, making "good" and "evil" meaningless terms.
Generally agree with you on e.g. there being no free will, but I wouldn't discard the human instincts behind "good" and "evil" so quickly.
Good and evil are just terms to describe what you assign high positive or negative value to. You can base that on the objectively good and bad things that we know of (ironically), which are pain and pleasure, meaning subjective human experience. Pain, suffering, misery are pretty much defined by being undesirable, they are the tools evolution ended up using when it wants to make us
not do something. So yeah, while there is a little bit of wiggle room there (e.g. sadomasochistic tendencies), these things have limits. Crank the pain up high enough and no one wants to experience it any more. It's reaches a level where it is objectively
bad. Most people don't want to experience any of it. Same with pleasure and happiness, just the other way around.
So, there we have the plus and minus poles of our value system. They might not be perfectly objective, but they are as objective as we could get them. Which is much better than not having any "good" and "evil" because you couldn't get them perfectly right. Values are social tools, and tools don't have to be perfect, just fit for purpose.
Although these terms are too narrow to encompass everything the feeling inside your heart when you watch your kid grow old to the taste of a good sandwich. So we might want to use "utility" instead.
That would be my middle way. "Good" and "evil" are present in every value system, even if the exact terms aren't always used. It's just about agreeing on what is valued positively and what is valued negatively and how much so. And since many people are not gifted with enough mental capabilities to think these things through, we can keep using these more natural, more emotionally loaded phrases like "good" and "evil" to build a bridge between the more and less intellectually gifted.