The very first Pope, Saint Peter, was apparently martyred and crucified in the same manner Jesus was said to have been. The story is, Saint Peter felt he was unworthy of dying the same way Jesus did so he requested to be crucified upside down.
I guess Saint Peter was the first one to be metal as fuck, yeah? Not likely.
The Church Of Satan adopted this particular symbol and claimed it as a symbol of the Antichrist mostly out of a desire to be opposite of it’s Christian & Catholic counterparts, which is how it became indicative of Satan.
But those that know it’s true origins are probably very confused with Anti-Christians, Satanists, and metalheads toting inverted crosses on their clothes and tattoed on their skin.
Think it’s evil? Quite the contrary. Roman Catholics claimed it first.