The Greek word translated by the NIV as hell is Tartarus.
This word was used in the Greek mythology with reference to the underworld, a place of torment where the Titans were imprisoned. Hell
is thus not a good translation of this word. We read only of angels being cast into hell in Revelation 20:7–15, though angels are bound in Revelation 9:14. Ultimately the reference to Tartarus and being placed in gloomy dungeons is meant to convey the idea that the angels or messengers have been secured for final judgment. They will not escape God’s wrath on the day of judgment. As their judgment is certain, so too the judgment of the unrighteous and the ungodly (2 Peter 2:9; 2 Peter 3:7).1
4 For if God did not spare angels when they sinned, but cast them into hell and committed them to chains of gloomy darkness to be kept until the judgment;