As his pickup truck roared toward the Todt Hill section of Staten Island, Anthony Comello had no intention of killing a mob boss.

He wanted only to arrest him, his lawyer said in new court documents.

The details of what happened next are well known. Mr. Comello, 24, arrived at the home of Francesco (Franky Boy) Cali, a leader in the Gambino crime family, and lured him outside. The men spoke briefly. Then prosecutors say Mr. Comello pulled a gun from his car and shot Mr. Cali, leaving him to die in the street.

Mr. Comello, an aimless young man who lived with his parents on Staten Island, was arrested three days later and charged with murder.

But court documents filed on Friday offered a glimpse into the deeply troubled mind of Mr. Comello, who his defense lawyer says was so deluded by internet conspiracy theories that he was determined to conduct a citizen’s arrest of Mr. Cali and turn the Mafia leader over to the military.