9:11 They have as king over them, the angel of the abyss; his name in Hebrew is Abaddon, and in the Greek he has the name Apollyon.

They have as KING over them, the ANGEL of the ABYSS;

Solomon rightly observed that animal locusts do not have a king in Proverbs 30:24-28. Therefore, this phrase clearly indicates that these are other than animals. To have an angel (fallen) leading them indicates their spiritual nature, that is, demons. There are more creatures than just satan who are evil or fallen, Matthew 25:41.

It is not satan himself even though he rules "principalities" and "power" as found in Ephesians 6:12 and Colossians 2:15. And satan's place currently is not in the abyss but in heavenly places as shown in Ephesians 2:2.

his name in Hebrew is Abaddon, and in the Greek he has the name Appollyon.

John is the only writer who uses bilingual terms. He does here and in his gospel (John 6:1; John 19:13,17,20; John 20:16). This is his style of writing; however, here in this verse he may have wanted to emphasize that this demonic locust plague was going to affect all "unsealed" both apostate Jews as well as unsaved Gentiles.

The term 'ABADDON' means 'destruction' or 'ruin'. It is used in Job 26:6 as the place of destruction, and even this place is totally revealed before Christ Jesus (Proverbs 15:11). There is no comfort there, even as in Sheol, Proverbs 27:20.

The term 'APOLLYON' is the name of a person meaning 'destoyer'. This personifies the leader of the fallen angels who are released as these awful stinging locusts -- demons.

It was accepted in New Testament times that there were spirits who were imprisoned. Peter eludes to them in 1 Peter 3:19 and 2 Peter 2:4. Abaddon apparently is chief among these imprisoned spirits. Also see other references to the destroyer in Jeremiah 4:7, Jeremiah 6:26, Isaiah 16:1-5, Daniel 8:23-26, Daniel 9:26, and Daniel 11:44.