One interesting question is why the Devil sinned in the first place? It isn’t the same as the problem of Evil, as we already know that the Devil has Free Will, but in a good Creation and with the perfect example of God, before all corruption that came precisely because Original Sin, how could “he” choose evil? How could “he” be prideful?
We can trace back the origin of sin and evil in “him”, as “he” tempted Adam and Eve before they sinned, but I haven’t heard any theologian explain why “he” sinned more than saying “because of Free Will”.