God is unbending in regard to His law. Peter shows this by illustrating that it does not matter who sins or when he sins. Before God ever created man, angels broke the law of God. God, being just and holy, could do only one thing. Because He cannot permit sin to abide in His Kingdom, He had to follow through with the punishment.
John W. Ritenbaugh
The Covenants, Grace and Law (Part 16)
"Hell" comes from the Greek tartaroo, and it means "a place of restraint." God did not spare the angels, but He cast them down to a place of restraint, a kind of prison.
In Greek mythology, Tartarus was the lowest hell, the place where the Titans (who were defeated by Zeus) were restrained. It is described as being as far below Hades as heaven is high above the earth. As far as we can apply Greek mythology, we can understand that these angels were cast so far down as to be out of sight. Their place of restraint was so far down that one would think they would never be able to crawl out.
God is trying to get across that the angels have been defeated—cast down from heaven to the earth, as Revelation 12 shows. The earth, then, is a place of restraint, a prison, for them.
To add to the imagery, they are bound in "chains of darkness." This amplifies the thought that Peter is making: The demons are restrained. There is some disagreement among scholars whether Peter uses the word that is translated here as "chains" or whether he means "silo." Almost everyone understands what a silo is. To an American, it is a tall, cylindrical object in which grain is stored. To the Greek, a silo was an underground pit where grain was stored. Whether it is a chain or a silo, it does not matter. God is trying to assure us that the demons have been restrained.
They are being restrained because they are facing judgment. Unfortunately for us, they are restrained in the place where we live! The earth is the silo, the storage bin. We are sharing this place with them. Worse, as they would see it, we are intruders in their space. They consider us invaders.
John W. Ritenbaugh
Satan (Part 1)
When God cast them back down to earth, He placed restrictions on their powers and limited them to "their proper domain" or "first estate," that is, the earth. Here, they await their judgment for their rebellion. "Hell" in II Peter 2:4 is tartaroo, a place of restraint for the wicked. Though Satan himself may appear before God's throne in heaven, he and his demons can do only what God allows (Job 1:6-12; 2:1-7).
Richard T. Ritenbaugh
Basic Doctrines: Satan's Origin and Destiny