Traditionally, the upside down cross of St. Peter is supposed to represent his humility. But inversion in occult symbology always carries double meaning.
The inverted symbol represents not humility, but reversal of spiritual flow. The descent of the divine into the lower realms, or it's corruption.
In Hermetic language, it's the fall of spirit into matter.
In Luciferian or Saturnian currents, the inversion is a deliberate act of claiming dominion over the material plane. "As above so below", but inverted "As below, so above"
Thus Peters inverted crucifixtion could be viewed not as humility, but as ritual inversion, marking the transfer of spiritual authority. The beginnings of the Churchs control over divine revelation.
This would explain why later Saturnic, or Satanic groups would reclaim the inverted cross: it was already the true sigil of the Church, the mark of Fallen Gnosis, not it's opposition.
Peters canonization by Rome legitimized the structure that would later obscure the inner mysteries under dogma and fear.
Peter is a ritual figurehead through whom the transition from Christic Revelation to Roman Dominion was made manifest.