John was an apostle of Jesus Christ who wrote many of the texts that inspired the Book of Revelation after he received a preview of the end of days. It's from John's writings in Revelation that we learn the angel of death is not the devil, but Abaddon, the leader of fallen angels tasked by God to torture Earth and humanity as punishment for the sins of humankind.
Abaddon's role in the Old and New Testaments provide a horrifying look at the part archangels are said to play during judgment day, and the function of such angels and demons doesn't sit well with the Christian view of an all-loving and merciful God.
Even more disturbing, in the Gnostic texts, Abaddon is pivotal in the creation of humankind, gathering the dirt from which God created Adam. So it's perhaps fitting that Abaddon will also gather souls and carry them to the place of God's final judgment. While the concept of Satan is frightening, it's nothing compared to an angel of death commissioned by God to torture sinners.