A.T. Robertson Commentary Revelation 9:11

"They have over them as king the angel of the abyss: his name in Hebrew is Abaddon, and in the Greek [tongue] he hath the name Apollyon." — Revelation 9:11 (ASV)

As king (βασιλεα). Predicate accusative and anarthrous. In Pr 30:27 it is stated that the locust has no king, but this is not true of these demonic locusts. Their king is "the angel of the abyss (verse 1) whose orders they obey."

His name is (ονομα αυτω). "Name to him" (nominative absolute and dative, as in 6:8).

In Hebrew (Εβραιστ). Adverb as in 16:16; John 5:2; 19:13,17,20; 20:16. Αβαδδων. A word almost confined to the Wisdom books (Job 26:6; Psalms 88:11; Proverbs 15:11). It is rendered in the LXX by Απωλεια, destruction.

In the Greek tongue (εν τη Hελληνικη). With γλωσση or διαλεκτω understood. As usual, John gives both the Hebrew and the Greek.

Apollyon (Απολλυων). Present active masculine singular participle of απολλυω, meaning "destroying," used here as a name and so "Destroyer," with the nominative case retained though in apposition with the accusative ονομα. The personification of Abaddon occurs in the Talmud also. It is not clear whether by Apollyon John means Death or Satan. Bousset even finds in the name Apollyon an indirect allusion to Apollo, one of whose symbols was the locust, a doubtful point assuredly.