He was supposed to bring light and continue God's work. He was the main, most powerful archangel, God's favorite son.
He hated Adam and humans, and didn't want to aid humanity in any way or form.
In the Book of Revelations 12:7 he rebelled and started an all-out war against God and all the archangels that supported him like Michael, and was cast out of heaven when he lost that war.
Then he was doomed to stay in hell and torment other souls that come in there until the end of time.