” I beseech you the executioners, crucify me thus, with the head downward and not otherwise: and the reason wherefore, I will tell unto them that hear.” ~ Saint Peter (Acts of Saint Peter, ch. 37; trans. M.R. James)
Saint Peter, our first pope, spoke these words to his Roman executioners when he was about to be crucified. He deemed himself unworthy to die in the same manner as Our Lord Jesus Christ and requested to be crucified in a more humiliating manner in love of Him. These words are recorded in a 2nd century document which was later the basis of the Acts of Saints Peter and Paul which records Peter saying he wanted his feet pointing to Heaven.
It was not until recently that Saint Peter’s Cross became affiliated with Satanism and that was due to Hollywood’s using it in exorcism movies. Long before that it was a symbol of Saint Peter, but fanatical anti-Catholics like to use the Hollywood narration and promote it as fact. It should not be a surprise that Satanism intentionally uses Christian symbols and mocks them in addition to making mock sacraments. Though I could see some sort of negative view of the upside-down cross since Saladin paraded the True Cross upside down through Jerusalem after he conquered the holy city before destroying the True Cross as a sign of rebellion against the Cross of Christ.
However, you could say it depends on the intent of the artist. You still cannot say whenever the Church depicts Saint Peter’s Cross it must mean the Church is Satanic and opposes Christ. The Church depicts it because it calls upon our duty to humble ourselves before Christ as Saint Peter did on the cross while submitting to his successor the Vicar of Christ. The Mamertine Prison is also a church and as legend says is the place where Saints Peter and Paul were imprisoned; Saint Peter’s Cross can be seen on the altar within the Mamertine Prison.
You can find pictures here: http://www.padfield.com/italy/mamertine-prison/index.html