It seems pretty clear that the angel who fell, becoming Satan, was far from perfect.
I am not aware of any bible verse that indicates he was "created as a perfect being" as you say.
Also, he thought himself equal to God. He did not "stand up against the oppressor", he was cast down from Heaven into the earth, where he remains as god of this world.
God gave free moral agency to him and all the angels. Satan took 1/3 of the angelic host with him - they now serve here as demons.
God gave the same free moral agency to you and me. The difference between us and the angels is that we are created in God's image, and they are not. Also, they only exist in the spiritual world, where as we are spirits who live in a physical body.
We are higher than the angels, and they were created to serve us and God.
While you are here, you have a choice; to serve God or serve Satan. You can not serve both.
If you read the end of the book, you can see which one wins.
Not surprisingly, it's the one who wrote the book.
(The one who wrote the book also created the universe, space, and even time itself - so He does have an advantage!)