God doesn’t define what is good/evil/etc., the users of language do
I’ve seen Christians claim on many occasions, including on this forum, that “God defines what is good and evil”, “God is the definition of goodness”, “God is good by definition”, or similar.
Simply put, that’s not how definitions work.
Language is used by humans to communicate, and it is only in the context of language that definitions make sense in the first place. The meaning of words is of course not always universally agreed upon, and is also subject to change over time, but broadly speaking, the “definition” of a word is just what most users of the language understand that word to mean. The editors behind almost every modern dictionary agree with that idea, which is why dictionaries today are usually descriptive (they describe how people use language) rather than prescriptive (they describe how people should use language).
Indeed, the only way to prescriptively “define” what “good” means is to change the minds of the people who use the word “good”. Words have no meaning outside of language, and language only exists in the minds of the people who use it.
Since Christians believe that God does not just “change the minds of people” because of free will, it follows that God has no influence on the meaning of “good”, “evil”, and similar terms, and is most certainly not their definition.
God can decide that a “dog” is an animal that flies around and lays eggs. That doesn’t make it so, as long as billions of humans understand “dog” to mean a four-legged animal that barks.
Just the same, God can decide that “good” means “like me”, and “evil” means “unlike me”. As long as people associate certain behaviors with the words “good” and “evil”, He hasn’t changed the meaning of those words at all. At best He has created something like a new language, in which commonly understood terms somehow mean different things than they normally do. Language is always defined by its users, never by an authority.
In fact, I’d wager that even most Christians have their own subconscious definitions of “good” and “evil” that flat out contradict the idea “God = good”. For example, if you ask a Christian whether it is good or evil to indiscriminately kill millions of people, they will likely answer “it is evil”, not “it is evil, except when God did it with the flood”. People have an innate understanding of what good and evil are, shaped by their own experiences and by the society they live in. That is the definition of “good” and “evil”, not what God thinks it should be.