Studying the Book of Revelation can help us understand which angels in the prophecies of this book are godly and which are not. From the book we can learn that basically there are three ways by which angels can come down from heaven to the earth.
Firstly, an angel may be forcefully cast down from heaven to the earth. Speaking of his intial fall, Jesus said in Luke 10:18 that He, "...beheld Satan as lightning fall from heaven". Another example of this is found in chapter 12 of Revelation, where war broke out in heaven between the forces of the archangel Michael, and those of Satan. The war ended with Satan and his angels being forcefully expelled from of heaven for the last time. All of those thrown down at that time are wicked angels.
Secondly, angels can descend from heaven with controlled descent, as shown in chapters 10:1 and 20:1-2. The context of these verses shows us that these are holy angels executing the will of Almighty God against the wicked.
Thirdly, an angel may come down from heaven to the earth because the element or condition which had sustained his position in the heavens was removed, causing him to fall to the earth. In a manner of speaking, the rug was pulled from under his feet. That is the case with the star (or angel) of chapter 9. In Scripture, it is no where apparent that God deals with holy angels in such a fashion. So it is clear that the star (or angel) of chapter 9 is not a holy angel. Exactly who he is, or may be, will become more clearly understood as we continue this study.
The "key" which is given to the star (angel), may simply represent the words necessary, when spoken, to cause the bottomless pit, to be opened. Jesus stated in Rev. 1:18 that He has the "...keys of hell and of death..."
In I Thess. 4:16 we are told that the dead in Christ will be raised with a shout. That shout (or word) is the key which will open the graves (Hades) at that time; and the blast of the trumpet, which follows, will summon those in the opened graves to come forth.
This same terminology may be applied the the angel of chapter 20, who is charged with casting Satan into the bottomless pit and closing it up for a period of one thousand years. Evidently the pit will remain open during the tribulation period, as the Beast (Antichrist) will go into it briefly, only to ascend from it and begin his absolute reign of terror and conquest.