When the Devil rebelled against God, the rebellion was not just Him alone.
Perhaps as many as a third of all the Angels. Like the Devil, they had free will and chose to leave God and follow Him.

These rebellious (or 'fallen' angels) are known as demons, evil spirits or unclean spirits.

They now serve the Devil and, like him, are evil and hate God and all people.
They enjoy and want to cause pain, misery and suffering to people and many time they are the cause of physical and mental illnesses.

Remember, demons were once God's Angels and so they can make themselves look attractive and claim to have more knowledge than they really do, but they only want to cause misery and chaos.