Angels are NOT robots. They are moral creatures
who can make moral decisions.

In this way they are like men. They, just like men, are made so that they are able to choose to NOT DO what they are supposed to do, or TO DO what they are not supposed to do. Also like us, angels are often allowed, beyond just choosing (within limits set by God), to do things which they should not. So it says of God,

“Behold, he puts no trust in his servants.
He charges his angels with error. ”
Job 4:18 WEB

So we saw in Part III “the prince of Persia” in Dan 10:13, and in that passage he is opposing the archangel Michael as he is trying to come to the aid of the prophet Daniel.

“15 Behold, he puts no trust in his holy ones;
Yes, the heavens are not clean in his sight:
16 How much less one who is abominable and corrupt,
A man who drinks iniquity like water!”
Job 15:15-16 WEB

When angels have sinned they (again like men)
are often punished WITHIN TIME.

Peter argues that if God did not even spare the angels, how can anyone escape his sins.

“For if God didn’t spare angels when they sinned, but cast them down to Tartarusa, and committed them to pits of darkness, to be reserved to judgment; ”
2Pe 2:4 WEB

Similarly, Jude speaks of these things.

“Angels who didn’t keep their first domain, but deserted their own dwelling place, he has kept in everlasting bonds under darkness for the judgment of the great day.”
Jude 6 WEB

The leader of open revolt was one called
Satan: the adversary.

Satan is spoken of as a cherub who was close to God in Ezekiel 28, as has been noted. Isaiah 14 is about the King of Babylon descending into torment, and the king of Babylon is treated as symbolic of the devil or Satan. It uses language here which it too exalted to speak of a mere man. It calls the one we call the devil or Satan as formerly being the “day-star, son of the morning!” Isa 14:12 WEB. This cherub tried to become THE GOD, and failed, and will be cast down to destruction.

“3 You said in your heart, I will ascend into heaven, I will exalt my throne above the stars of God; and I will sit on the mountain of congregation, in the uttermost parts of the north; 14 I will ascend above the heights of the clouds; I will make myself like the Most High. 15 Yet you shall be brought down to Sheol, to the uttermost parts of the pit. ”
Isa 14:13-15 WEB

But how blessed are those angels who are
faithful to God.

20 “Praise Yahweh, you angels of his,
Who are mighty in strength, who fulfill his word,
Obeying the voice of his word.
21 Praise Yahweh, all you hosts of his,
You servants of his, who do his pleasure.”
Psa 103:20-21 WEB

Not all do, but most of them do.

WEB is the World English Bible, a copyright free revision
of the original ASV American Standard Version 1901