If Satan was once a "perfect angel" how did he become evil and imperfect?

Ezekiel 28 :15 says that Satan’s ways were perfect until wickedness was found in Him. The Hebrew word for wickedness comes from a root meaning “to act wrongfully or unjustly”, so the wickedness was found by observing his actions. I think this implies that angels as well as humans have agency, or free will to choose their behavior. Although Satan was the first to rebel he was by no means the last. All of us have the potential for evil.