they had a king over them
These locusts are to be contrasted with natural locusts which have no king: The locusts have no king, yet they all advance in ranks (Pr. Pr. 30:27). These demonic locusts have a king and advance like their natural counterparts:
They run like mighty men, they climb the wall like men of war; every one marches in formation, and they do not break ranks. They do not push one another; every one marches in his own column. Though they lunge between the weapons, they are not cut down. (Joel Joel 2:7-8)In one of the translations of the LXX, Amos is shown a king over a swarm of locusts: Thus has the Lord [God] shewed me; and behold, a swarm of locusts coming from the east; and, behold, one caterpillar, king Gog (Amos Amos 7:1).1 Perhaps because the plague of locusts was seen by Amos as a sign of Gods judgment (Deu. Deu. 28:38, Deu. 28:42; Amos Amos 4:9; Joel Joel 1:1-7), Gogthe archenemy of Israelis pictured as their king (Eze. Eze. 38:2-3, Eze. 38:14, Eze. 38:16, Eze. 38:18, Eze. 38:21; Eze. 39:1, Eze. 39:11; Rev. Rev. 20:8+).
Ἀβαδδών [Abaddōn] , a transliteration of the related Hebrew term. The Hebrew term אֲבַדּוֹן [ʾăḇaddôn] is derived from אבד [ʾḇḏ] meaning to perish, become lost, be ruined. It is translated by destruction and associated with שְׁאוֹל [šeʾôl] in Job Job 26:6 and Pr. Pr. 15:11, where it denotes a place which is seen by the omnipresence and omniscience of God. Abbadon is personified along with death (מָוֶת [māweṯ] ) as having heard of the fame of elusive wisdom (Job Job 28:22). It is said to be the destination of a consuming fire (Job Job 31:12) and is associated with the grave (קֶבֶר [qeḇer] ), but differentiated from it (Ps. Ps. 88:11). It is said that hell (שְׁאוֹל [šeʾôl] ) and Abbadon (destruction) are never full (Pr. Pr. 27:20). In all of these uses, it denotes a location which is associated with Sheol and the grave, but differs from them. The rabbins have made Abaddon the nethermost of the two regions into which they divide the lower world.3
Some suppose him to be Satan:
The king of these locusts is named in both Hebrew and Greek. The name that God gives to him is Abaddon or Apollyon. . . . In it, of course, is one of the titles of Satan of which there are so many in the Bible. We are reminded of the two passages in Matthews Gospel where the Lord Himself speaks of the prince of the demons or rather where He comments on the Pharisees use of the name, Beelzebub, whom they call the prince of the demons. . . . The Lord said, commenting on the Pharisees thought, If Satan cast out Satan, he is divided against himself (Mtt. Mat. 12:26), thus linking the name of the one they called prince of demons to Satan himself.4However, if Satan is taken as the fallen star to which the key is given to open the pit (presumably from the outside), and this angel is from the pit, it argues against his identification as Satan. Perhaps this angel is the highest ranking angel which has been confined within the pit for the duration of their demonic captivity? He is probably not related to the elect angel who eventually locks Satan in the pit (Rev. Rev. 20:2+).
John gives his title as the angel of the abyss. Some identify this angel as Satan, but his domain is the heavenlies (Eph. Eph. 6:12), where he is the prince of the power of the air (Eph. Eph. 2:2). He is not associated with the abyss until he is cast into it (Rev. Rev. 20:1-3+). This angel is better viewed as a high-ranking demon in Satans hierarchy.5Others suppose this angel to be Antichrist:
It is the Destroyer of the Gentiles of Jer. Jer. 4:7, translated Spoiler in Isa. Isa. 16:4 and Jer. Jer. 6:24. Suitable name is this for the one who is the great opponent of the Saviour. Destroyer is close akin to Death in Rev. Rev. 6:8+. The reason why his name is given here in both Hebrew and Greek is because he will be connected with and be the destroyer of both Jews and Gentiles! But why give the Hebrew name first? Because the order in judgment, as in grace, is the Jew first - see Rom. Rom. 2:9 and Rom. 1:16 for each, respectively.6The beast that makes war against the two witnesses is said to ascend out of the bottomless pit (Rev. Rev. 11:7+; Rev. 17:8+). His coming is according to the working of Satan, with all power, signs and lying wonders (2Th. 2Th. 2:9). His ascent from the pit may speak of his physical origin, the source of his spiritual empowerment, or both. It appears his ascent from the pit takes place when his deadly wound is healed (Rev. Rev. 13:3+, Rev. 13:12+) for John implies the order of events: The beast that you saw  was, and  is not, and  will ascend out of the bottomless pit and  go to perdition (Rev. Rev. 17:8+). It is unlikely that the Antichrist would be found in the pit at its initial opening, already having been emprisoned with demons for an extended period of time7 or that he would be said to be a king over demons.8 This king is an angel, but nowhere does Scripture refer to Antichrist as an angel. Instead, he is said to be a man (Dan. Dan. 7:8; Dan. 8:23; Dan. 11:37; 2Th. 2Th. 2:3). Admittedly, there is considerable mystery associated with the Antichrist, especially regarding his origin, how he is not, and will ascend and the means by which his deadly wound was healed (Rev. Rev. 13:3+). Yet, it seems unlikely that this king who is an angel refers to Antichrist. See commentary on Revelation 13 and The Beast.
This angel is probably neither Satan nor Antichrist, but a high-ranking fallen angel who has been king over the emprisoned demons for the duration of their time in the pit.
Ἀπολλύων [Apollyōn] , the Greek translation of the Hebrew אֲבַדּוֹן [ʾăḇaddôn] . The LXX usually translates Hebrew Abaddon as apōleia, destruction. Apollyon appears in the nominative case like a proper name would appear in quotes.9
Providing the name in both Greek and Hebrew is characteristic of John. John 6:1; John 19:13, John 19:17, John 19:20; John 20:16).10 It is another piece of evidence in favor of authorship by the Apostle John. See Authorship. John uses both names to emphasize his impact on both ungodly Jews and Gentiles.11
1 Οὕτως ἔδειξέν μοι κύριος καὶ ἰδοὺ ἐπιγονὴ ἀκρίδων ἐρχομένη ἑωθινή καὶ ἰδοὺ βροῦχος εἶς Γωγ ὁ βασιλεύσ. [Houtōs edeixen moi kyrios kai idou epigonē akridōn erchomenē heōthinē kai idou brouchos eis Gōg ho basileuṣ] Septuaginta : With Morphology (Stuttgart, Germany: German Bible Society, 1996, c1979), Amos 7:1.
2 It is possible that this is an attributive or descriptive genitive, but genitive of source indicates origin more than character and hence seems more appropriate in this context.Daniel B. Wallace, Greek Grammar Beyond the Basics - Exegetical Syntax of the New Testament (Grand Rapids, MI: Zondervan Publishing House and Galaxie Software, 1999, 2002), 110. Metonymy of the Adjunct. The name of the pit is given to the angel of the pit, by this figure, by which the abstract is put for the concrete.Jerome Smith, The New Treasury of Scripture Knowledge (Nashville, TN: Thomas Nelson Publishers, 1992), Rev. 9:11.
5 John MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 1999), Rev. 9:11.
7 Unless we adopt the theory of some that Antichrist already walked the stage of history, was slain, and will be revived yet future. Some consider a revival of Judas as possible fulfillment, but the Antichrists deadly wound is by the sword (Rev. Rev. 13:14+), whereas Judas committed suicide by hanging (Mtt. Mat. 27:5).
9 Although ancient Greek did not have the convention of quotation marks, it could express essentially the same idea with a nominative of appellation.Wallace, Greek Grammar Beyond the Basics - Exegetical Syntax of the New Testament, 60.
11 MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary, Rev. 9:11.