I don't think the idea of good and evil from historical times is the same as it is now; there are similarities at the basis of it all, but I don't think that they are ever truly defined (just my opinion) as most of it was defined by societal tradition. At the same time though in the present, it's our interpretation of societal norms that gives us a basis (or in your case, the Bible) for understanding good and evil. It's only because we've had different examples where we were able to understand the distinction. If every being was cruel, there would be no such thing as cruelty like, oh say, mother nature who is blind to such definitions - there cannot be light without darkness. That is, by default, moral relativism is it not? It's all basis of perception.
I love this kind of discussion.
First, the idea of good and evil is as old as humanity. If there is no God, meaning there is no transcendent source of morality, then it is personal or societal preference.
Second, nature is amoral. The hurricane does not pick which city it is going to devastate. A lion is amoral. It kills to eat.
Third the world was created with duality and humans were given free will. If we did not have free will we would be lima beans. You could argue (accepting God for arguments sake) that since people have a proclivity to do evil, God should not have created them. I can accept that argument. Because there is duality, does not mean it's all relative.
Ultimately, for the believer in God, the moral laws are laws because God said so. If Evilolive says torturing animals is good and Old Timer says torturing animals is evil. How do we decide? By majority rule? By physical or military force? If God says torturing animals is wrong, it's wrong. If the source or morality is transcendent, though evil may triumph from time to time, it is still evil.
What you say is 100% true, if there was no evil and all was good, life would be meaningless.
Up until Judaism, what you say is true. Whatever your society decided was OK was OK. For better or worse, Judaism changed that and it can't be changed back. Judaism said, it was wrong to expose babies to kill them because they were deformed or ugly. Judaism said it was wrong to torture animals. Judaism said it was wrong to oppress the stranger and the list goes on...
How are you going to teach your children right from wrong if there really is no right and wrong?
And I return to Hitler because it's such a fat target. For the believer, despite the fact the Nazi dominated society approved of Hitler's action, his actions were morally wrong. It is wrong to burn people alive, it is wrong to perform medical experiments on human beings. It is wrong to gas people and burn their bodies. It can never be right to commit murder. Murder and killing are different by the way. Like squares and rectangles. All murder is killing, but not all killing is murder. The commandment says Thou shalt not commit murder.
Can you can justify the killing of 12 million people by the Nazi's, 20 million by Stalin and 50-75 million by Mao on moral grounds? Can anyone say this was good and not evil? Only without God.