What Scripture tells us
The Bible gives only a few direct statements about fallen angels. God created all things, including the unseen powers of heaven, through and for Christ (Colossians 1:16). Yet some angels later sinned, and God placed them under judgement rather than overlooking their rebellion (2 Peter 2:4).
Jude says these angels did not remain in the position appointed to them, but abandoned it (Jude 1:6). That description presents their sin as a real act of rebellion. They were God's creatures, not evil beings equal to God from the beginning.
Revelation depicts conflict in heaven involving the devil and his angels, with Michael and his angels opposing them (Revelation 12:7-9). Some Christians understand parts of Revelation's imagery differently, but its central point is clear: the devil is not God's equal, and rebellious spiritual powers remain under God's authority.
For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
What the Bible does not explain
Scripture never gives a detailed account of the first angelic sin or explains why God chose not to prevent it. That can feel unsatisfying. Still, an honest answer should not go further than the Bible does.
People often speak about angelic free will, a test in heaven, or a specific sequence of events before humanity began. Those ideas may try to make sense of the question, but the Bible does not lay out that full story. We can say that angels were responsible for their rebellion, but we cannot confidently map every reason behind God's permission of it.
This mystery is not unique to angels. The Bible repeatedly shows God ruling over a world where his creatures make choices for which they are accountable. It also insists that God's character stays clean and good.
God did not approve of their sin
Allowing something is not the same as causing it or delighting in it. James says God is not drawn towards evil and does not entice anyone to do wrong (James 1:13). John describes God as completely free from darkness (1 John 1:5).
So the angels' sin cannot be blamed on a flaw in their Creator. God judged it. Their judgement matters because it shows that spiritual rebellion is neither harmless nor beyond God's notice (2 Peter 2:4; Jude 1:6).
God's patience with evil for a time should not be mistaken for weakness. He is able to limit it, use even human and spiritual opposition within his purposes, and bring just judgement in the end. The Bible does not ask us to call evil good in order to trust God.
Evil will not have the last word
The devil may oppose God, accuse people, and deceive, but he is a defeated enemy, not a rival king. Revelation looks ahead to his final judgement (Revelation 20:10). God's rule has never been in danger.
For people, the centre of hope is Jesus. He came to destroy the works of the devil and to bring us back from sin into God's life (1 John 3:8). The fall of angels is sobering, but it is not the final message of Scripture.
If this question leaves you with unanswered thoughts, that's understandable. Read the passages slowly and bring the question to God in prayer. What is clear is enough to hold onto: God is holy, evil is real, rebellion is judged, and Christ's victory is sure.
And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.