Angels are actually pure spirits. The word “angel” means messenger.
There was once a beautiful angel, in fact a chief angel, called Lucifer, the light-bearer.
He became very proud, despite the fact that he was only a creature. He wanted to be like God. Because of this, he rebelled against God, who is infinitely good. Many other angels were deceived by him and rebelled against God. In fact, they chose to hate God.
There was a war, described in the last book of the Bible, in which the chief angel Michael lead the angels who served God in battle against the rebellious angels led by Lucifer - who became known as Satan, which means “adversary”. Satan and the fallen angels were defeated. The Lord Jesus said: “I saw Satan fall like a lightning.”
There is no such thing as a “balance of good and evil”. God is infinite and omnipresent, and God is infinitely good: there is no evil in Him. So, while evil may seem to be more visible (like a loud noise is more “visible” than a soft tune), it is actually very, very small in comparison to the good.
The choice made by the angels who rebelled against God was a definitive one, because they weren’t like us: they were created to be perfect.
The rebellion of man did not arise from a free choice, but because the fallen angels tempted man to sin. Because of this injustice, Jesus (who before was purely spiritual) became man and came to save us. After He was born, every man feels inside his heart the desire to repent and to be with God. Some feel this desire more, others less. So all humans can repent, while alive.
After death, however, the last choice made by the person becomes like that of the fallen angels: a definite choice. He cannot change his mind. God will open His arms, but the person who chose to reject Him in life will reject Him and join the fallen angels. This is not what God wants, but because of our freedom we can do this, which greatly displeases Him.
This is the synthesis of the judgment: Christ came not to condemn sinners, but to save them. Those who are condemned, have condemned themselves. The apostle John wrote that this is the verdict: they chose to walk in the darkness.
Why does God, who is omnipotent, allow this? Because he made us free, “in His image and likeness”, not slaves, like robots. When someone is not your slave, but your friend, you do not force him to do or not to do something. If a friend argues with you and no longer wants to speak with you, you cannot force him to be your friend. All you can do is: remain his friend, invite him to return whenever he wants, and if he wants to be your friend again, accept his friendship.
I hope some of this sounds helpful
