If God created the devil as a perfect angel, why did he become a sinner?

“How YOU HAVE FALLEN FROM HEAVEN, O morning star, son of the dawn! YOU HAVE BEEN CAST DOWN TO THE EARTH, you who once laid low the nations! You said in your heart, ‘I will ascend to heaven; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of the sacred mountain. I will ascend above the tops of the clouds. I WILL MAKE MYSELF LIKE THE MOST HIGH.’”
—Isaiah 14:12-14.

The being who became the devil was originally called Lucifer, meaning “day star” or “shining one.” Somehow, in this angel’s heart, vanity and ambition began to take the place of devotion. The seed of pride grew into an obsession to actually take God’s place. God had already given him a very high position in heaven as a leader of the angels. But he wanted even more. Growing more and more jealous of God, he sought to seize God’s throne.

Lucifer must have worked very hard to persuade other heavenly beings that he had a better plan. It is easy to imagine Satan arguing that God was holding something back from them, and that the divine law was too restrictive, and that God was an uncaring sovereign. He slandered the name of the One whose character defines what love is.

How was this conflict in heaven resolved?

“Your heart became proud on account of your beauty, and you corrupted your wisdom because of your splendor. So I threw you to the earth.”
—Ezekiel 28:17.

When Lucifer’s heart became proud and he chose to rebel, God had to cast him out of heaven. Why? Because pride and selfishness, which are sin, and at the root of all sin, cannot exist in the presence of God.

Lucifer twisted love for God into an unhealthy love of self. Jealousy changed the chief of angels into the devil or Satan. And in order to preserve the peace and harmony of heaven, he and one third of heaven’s angels, those who joined him in rebellion, had to be expelled (Revelation 12:4, 7-9).