Former Muslim was zealous for Allah and even thought of going to Jihad so that he could die as a martyr to earn rewards in heaven. When he tried to convert Christians, he was asked to present his evidences and began to study deeper Islam’s allegations about Christianity. This was when doubts about Islam surfaced.
The events of Sep 11 was an important trigger, causing him to think of the atrocities he could have committed if he hadn’t stopped thinking of becoming a jihadist. It also shook his concept of Islam being a religion of peace. This was so different from what he learned from the Bible about forgiving our enemies.