Pride (Proverbs 16:18). Satan’s original name was Lucifer. He was the Guardian cherub in Eden (Ezekiel 28:14). He wore precious stones. He took notice of his beauty (Ezekiel 28:17). Seeing how beautiful he was, his pride took over, and he tried to replace God (Isaiah 14:13). Apparently, the angels had a freewill back then. Satan chose to sin. He got one-third of the other angels to follow him (Revelation 12:4). They all sinned and will face some dire consequences (Matthew 25:41).
Mankind also has free will. He can choose God or Satan. If he chooses God eternal life awaits him (John 3:16). If he chooses the world the same fate awaits him as Satan and the fallen angels (Galatians 5:19-21). It is pride that keeps people from inheriting eternal life.