Satan didn't start out bad.
God created Lucifer (Ezekiel 28:13, 15), a beautiful, shining angel who was a leader in heaven (Ezekiel 28:14; Psalms 99:1). One day, Lucifer began to think more highly of himself than he should (Ezekiel 28:17), and he began to question God's authority in his life. What began as thoughts in his own mind turned into public murmuring among the other angels.
This murmuring became louder and louder, until ultimately, the Bible tells us Lucifer wanted to remove God from the His heavenly throne and sit there himself (Isaiah 14:13, 14). He fomented a rebellion that drew one-third of the angels to his side and caused a war in heaven (Rev. 12:7-9)!