I think you missed the main point of Old Testament.
Perhaps the worst sin as highlighted in the Book of Genesis began in the Garden of Eden. And from that sin all other sin is derived. We want to be God. How was Eve tempted? "For God knows that when you eat from it your eyes will be opened, and you will be like God, knowing good and evil."
Don't you find it fascinating that the first sin, is behind every other permutation of sin? Isn't that what is motivating you to write this last post. You want to declare what is right or wrong... you want to "be God" and judge others.
I am not judging you, since I am guilty like you of being a sinner, I am only observing how "natural" sinning can be, especially judging others and deciding for ourselves what is good and what is evil.
Your whole post is about the desire of Christians to think they are God and then to judge others.
But your post does not define Christians, since haven't non-Christians killed millions of people from the Romans to the Russians, from the Mongols to the Chinese.
So, if Christianity is not the common thread behind evil, what is? The common thread is our humanity. God sent Jesus to save us from ourselves. Don't confuse the cruelty of man with the love of God.
Look how little compassion you showed in your post, yet the Bible (which details the history of "God's people" is with episode after episode of men sinning against God and their fellow man).
And yet, even after committing so much evil, God returns. God has not given up on man, including you.
We are all given a lifetime to determine what we want our eternal relationship with God to be. Instead, most of the time we run around pretending God put us in charge of determining what is right and what is wrong... that is true of all humans, Christians and non-Christians...
Which is why the saying that there are two kinds of people in the world... sinners and forgiven sinners is a better way to view Christians and non-Christians. We all share the sin that goes with our humanity.