MG
The devil is an angel who rebelled: ‘How you are fallen from heaven, O Day Star, son of Dawn!… You said in your heart, “I will ascend to heaven; above the stars of God I will set my throne on high…” But you are brought down to Sheol, to the far reaches of the pit’ (Isaiah 14:12–15).
He was not alone, other angels joined him: ‘Another sign appeared in heaven: behold, a great red dragon, with seven heads and ten horns, and on his heads seven diadems. His tail swept down a third of the stars of heaven and cast them to the earth… Now war arose in heaven, Michael and his angels fighting against the dragon. And the dragon and his angels fought back, but he was defeated, and there was no longer any place for them in heaven. And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world—he was thrown down to the earth, and his angels were thrown down with him’ (Revelation 12:3–9).
God has not destroyed the devil because He is merciful and wants him to repent: ‘As I live, declares the Lord God, I have no pleasure in the death of the wicked, but that the wicked turn from his way and live’ (Ezekiel 33:11).
Ed
God has given us freedom of choice, whether to obey Him or not. There is no indication in the Bible that the angels have that freedom of choice. They’re always referred to as God’s servants who do His will (for example Psalm 103:20, Psalm 104:4).
Death is the consequence of sin (Romans 6:23); the angels are immortal (Matthew 22:30), therefore they don’t sin.
Some people assume that Isaiah 14 is referring to a fallen angel. However, if you look at the context (v. 4) it’s clear that this is actually a colourful and symbolic prophecy about the king of Babylon.
Likewise, some people assume that Revelation 12 is a description of Satan being cast out of heaven in the beginning. But there are problems with this assumption. Revelation is a prophecy of things that were in the future when the book was written (Revelation 1:1)—not things that were in the past. Revelation 12 is a complicated vision which, along with the rest of Revelation, draws heavily on symbology from the rest of the Bible. If part of the vision is describing a pre-history angel rebellion, it’s very difficult to imagine what the rest of the vision could be about. (According to a common interpretation, chapter 12 is actually a prophecy of the Christianisation of the Roman Empire in the 4th Century ad.)
God is supremely merciful. The Bible tells us a lot about His mercy towards humans. It says nothing about His mercy towards fallen angels. The devil is going to be destroyed (Revelation 20:7–10). I believe the devil is a picture the Bible uses for human sinfulness. I suggest the passages you’ve quoted, along with all the other references to the devil in the Bible, make far more sense when you look at it like that.