You wrote: “One wonders whether the Devil pre-existed human beings, or whether he was a creation of the sins of mankind.” Here some related bible scriptures that widen the perspective.
Based on John8:44 scripture, it seems that Jesus also held the devil accountable for what happened.
John 8:44 You belong to your father, the devil, and you want to carry
out your father’s desires. He was a murderer from the beginning, not
holding to the truth, for there is no truth in him. When he lies, he
speaks his native language, for he is a liar and the father of lies.
About the origins of the Devil, there are some hints in the following bible scriptures:
Isaiah 14:12 How you have fallen from heaven, morning star, son of the
dawn! You have been cast down to the earth, you who once laid low the
nations! [NIV]
1 John 3:8 Whoever makes a practice of sinning is of the devil, for
the devil has been sinning from the beginning. The reason the Son of
God appeared was to destroy the works of the devil. [ESV]
Revelation 12:9 And the great dragon was cast out, that old serpent,
called the Devil, and Satan, which deceiveth the whole world: he was
cast out into the earth, and his angels were cast out with him. [KJV]
Isiah 14:12 calls him son of the dawn, 1 John 3:8 says that the devil has been sinning from the beginning. And Revelation 12:9 Mentions the devil (Satan) and his angels. Which could be used to support the idea, that Satan the devil, existed before humankind.
Another, somewhat more speculative idea is that Ezekiel 28:12-15, describing the King of Tyre, speaks also allegorically about Satan the Devil.
Note I have no Catholic background, and just wanted to provide some additional bible scriptures that can provide some additional insight.
But there seem no great contradictions in views on this point. Typically Satan the Devil is believed to be created together with the Angels in the beginning of creation.