How did the symbol of the upside-down Christian cross (which is St. Peter's cross in Christianity) first start to become associated with Satanism/anti-Christianity/evil in pop culture?
The wikipedia page currently says
The inverted cross is a recurring motif in metal music. One well-known example is Glen Benton of the band Deicide who branded an inverted cross onto his forehead.
Former Black Sabbath drummer Bill Ward suggested that the use of an inverted cross on the inner gatefold sleeve of their debut album by their label Vertigo may have been a promotional ploy or a misunderstanding of the nature of the group.
The Swedish metal band Ghost makes use of the inverted cross in place of the final "t" in the band's wordmark.
Many horror films use inverted crosses as part of their imagery and marketing, especially if the film involves demonic themes. Examples include The Conjuring and Paranormal Activity.
In the Chemin de la Croix by French composer Marcel Dupré, at the part where Simon of Cyrene helps Jesus carry the cross, an inverted cross motif appears at bar 7 and then twice more before it is replaced with a pastoral theme.
In the BBC television comedy series The Vicar of Dibley, Geraldine Granger is regularly seen wearing the cross as a pendant.
Chrollo Lucilfer and Shizuku Murasaki from manga series Hunter × Hunter sport the cross - Chrollo has it emblazoned on the back of his coat and Shizuku wears it as a necklace. Meme Oshino from the Monogatari series also has the inverted cross as an earring.