Satan
"Satan tempted man to sin. So Satan's defection must be more fundamental than man's, and human sin cannot be understood without mentioning it."
Contrary to popular opinion, it was the other way round. Man's sin resulted in the corruption of some angels, the inverting of the natural relationship of service between angel and man, and the transformation of Lucifer into Satan, the enemy of man's true interests.
(Or maybe angels are selves, and their world is their home as Earth is ours, and their service of us is mentioned as a relevant fact, not stated as a definition, in which case Satan is not one of them. Either way, the gist of what follows still applies.)
We have nobody to blame but ourselves. It was because the first couple's minds had already gone astray that Satan began to suggest how to give outward expression to their defection.
Man is the crown of God's creation. Eve's excuse that "The serpent tricked me" is as poor as Adam's excuse that "It was the woman's idea" (Genesis 3.12-13). In both cases, the responsible creature blames the subordinate.
Satan's fault consists of being "on man's side" (Mark 8.33), that is, a servant of man's defiance against God.
This is important because the idea that the demonic defection is more fundamental than man's supports the idea that the conflict between good and evil occurs primarily not in the world around us but elsewhere, in a spiritual realm that is separate from our world, and that man is only involved peripherally. This idea leads to a loss of concern for the real world and to the development of a morbid faith focused inward on itself. However much the centrality of man in God's purposes is then insisted on, it can make no headway against the overwhelming force of the idea that the spiritual is a separate realm.
According to the New Covenant Book, Christ often referred to Satan, but did not attribute to him a fundamental role in the great transaction that was to be accomplished. Satan does not appear in those texts on which his sense of his mission mainly drew, and is peripheral in the Hebrew classics in general. The other New Covenant Book writers agree with Christ in this. Certainly the New Covenant Book's understanding of Satan is very different from the earlier Hebrew view, but that should not lead us to forget the basic unity of viewpoint between the two groups of texts as regards the human condition and man's relations with God.