After the fifth angel sounded his trumpet in Revelation 9:1, a “star fallen it earth” is given a key to the shaft of the Abyss (NIV) or the “bottomless pit” (ESV, NRSV), which he unlocks and opens in 9:2. What does this star represent?
The star is usually identified as an angel based on Job 38:7 and other Second Temple texts which describe angelic beings as stars. But this angel could be a either a good or bad angel.
The key word is “fallen” (a perfect active participle from πίπτω). Robert Mounce (192) and R. H. Charles (1:238) consider the verb “fallen” to mean nothing more than “to descend” and his actions opening the shaft of the Abyss are no different than the angel who sounded the trumpet. Mounce suggests this is the same angel who locks the Abyss in Revelation 20:1. George Ladd argued the star “represents some angelic figure divinely commissions to carry out God’s purposes” (129).
It is quite true this star “fallen from heaven to earth” is fulfilling a divinely appointed task, but it is difficult to argue πίπτω simply means “descend” either in koine Greek or the book of Revelation. Things which fall are always bad in Revelation.
More often this star is identified with a fallen angel, either a powerful demon such as Abaddon, or Satan himself. Since Satan is described as falling from heaven in Revelation 12 it is possible this “star fallen from heaven to earth” is Satan. However, in Revelation 12:8 Satan is thrown down to the earth by Michael, a slightly different description than Revelation 9:1.
There are several examples in Second Temple period literature of “fallen stars” as demons. In the Animal Apocalypse, the writer says describes the origin of “bad animals” as fallen stars: “I observed the sky and behold, I saw many stars descending and casting themselves down from the sky” (1 Enoch 86:3). In the Testament of Solomon, a demon describes the activities of the “principalities and authorities and powers.” The demons deemed not worthy to enter heaven are “dropped like flashes of lightning to the earth.”
Testament of Solomon 20.14–17 I asked him, “Tell me, then, how you, being demons, are able to ascend into heaven.” 15 He replied, “Whatever things are accomplished in heaven (are accomplished) in the same way also on earth; for the principalities and authorities and powers above fly around and are considered worthy of entering heaven. 16 But we who are demons are exhausted from not having a way station from which to ascend or on which to rest; so we fall down like leaves from the trees and the men who are watching think that stars are falling from heaven. 17 That is not true, King; rather, we fall because of our weakness and, since there is nothing on which to hold, we are dropped like flashes of lightning to the earth. We burn cities down and set fields on fire. But the stars of heaven have their foundations laid in the firmament.
In the Coptic Apocalypse of Elijah both Elijah and Enoch confront the “shameless one” as fallen from heaven like the morning stars:
Apocalypse of Elijah 4.8–12 Are you indeed not ashamed? When you attach yourself to the saints, because you are always estranged. 9 You have been hostile to those who belong to heaven. You have acted against those belonging to the earth. 10 You have been hostile to the thrones. You have acted against the angels. You are always a stranger. 11You have fallen from heaven like the morning stars. You were changed, and your tribe became dark for you. 12 But you are not ashamed, when you stand firmly against God. You are a devil.
The Apocalypse of Elijah seems to have been written no earlier than AD 150 and is more likely was composed in the third or fourth century and is reliant on Revelation or other Christian apocalypses. However, the idea of Satan falling from heaven appears in Luke 10:18.
I do not think it is necessary to choose between a good angel who is dispatching his divinely appointed commission to open the Abyss and an evil angel who is acting on the orders of Satan to open the Abyss. Perhaps the angel is not (morally) either good or bad, but simply the being appointed by God to open (or lock) the Abyss.