The prophecy of Obadiah is a prophecy concerning Edom, that nation which descended from Esau, Jacob's brother. In order to understand the prophecy concerning Edom, one must understand all of the history of the nation, and its relationship to Israel and to God, from the days of Jacob and Esau.
The angels that left their first estate, left it because they decided to race-mix with men, and also with many other species, something we find only in apocryphal literature but which our Bibles as we know them today do not sufficiently explain. When Adam was placed into the Garden of Eden, that tree of the knowledge of good and evil – the results of that first rebellion against God – was already in the garden. These were a race of people (or angels if you must) who at one time knew good, and then knew evil – when they had rebelled against God. Their creation is not explained in Genesis, although Christ tells us in Luke chapter 10 and in the Revelation at chapter 12 that they fell “from heaven”.
The fall of Adam was the partaking by him and his wife of this tree of the knowledge of good and evil – they race-mixed with that person represented by the epithet of “serpent”. Cain was the result of this union, and in spite of the corrupted text we currently know as Genesis 4:1, it can be discerned in several other ways that Cain was not the son of Adam, although he was the son of Eve. Later on in the New Testament, but also often in the allegories of the Old Testament, are the descendants of Cain often referred to as “serpents”.