In 1983, Neill joined the United States Army. He met Robert Grady Johnson in February 1984 at a bar and became romantically involved. Neill was discharged in the summer of 1984 after disclosing that he was homosexual and quickly began having financial difficulties.  Neill and Johnson shared an apartment in Lawton, Oklahoma and a joint checking account at the Geronimo bank. The account had frequent checks returned due to insufficient funds. They were frequently at the bank to work out their money problems. The Geronimo Bank was a small facility, housed in a prefabricated building. It usually had only two tellers and no surveillance cameras or security guards. Neill commented on more than one occasion on the absence of the security measures and how easy he thought it would be to rob the bank.  On December 12, 1984, Neill shopped for guns at a local pawnshop but he was informed that a buyer must be at least 21 years old and Neill was only 19. On December 13, Johnson applied for a gun permit. That same morning, Neill went to a travel agent and tried to purchase flights to Nassau leaving at 6 p.m. the next day. On December 14, Neill and Johnson returned to the pawn shop and purchased a revolver seen on Neill’s previous visit.

Shortly after 1 p.m. on December 14, Neill entered the First Bank of Chattanooga in Geronimo, and forced the 3 tellers to the back room where he had them lie face down on the floor and stabbed them to death. The three employees (Kay Bruno, 42; Jerri Bowles, 19; Joyce Mullenix, 25) were stabbed a total of 75 times. Mullenix was six months pregnant.  Three customers entered the bank while Neill was attempting a decapitation of one of the tellers. The customers were taken to the back room and shot in the head. Ralph Zeller, 33, died from his wounds becoming the 4th and final murder victim. Bellen Robels, 15, and her husband Reuben Robels, 20, would recover from their head wounds. Neill attempted to shoot the couple’s 14-month-old daughter, Marie, but the gun was out of bullets.  Neill and Johnson were arrested on December 17 in San Francisco. Marked bills were used to pay for hotel rooms, limousine rides and shopping excursions. $3,700 were left on Neill and in the hotel room. Johnson was sentenced to 4 life sentences with the possibility of parole. Whether Johnson was in the bank at the time is in dispute. Neill testified that Johnson was at home waiting on him during the robbery. However, he had previously maintained that Johnson was with him in the bank. Marilyn Roach testified to hearing the voices of two men inside the bank.

Neill was sentenced to death twice, in 1985 and 1992. During the time of his incarceration, he converted to Christianity at first and then to Buddhism. On death row, Neill’s friendship with anyone was always doubtful, and his conversion to Christianity was often the topic of conversation, as his “friends” were skeptical even when he delivered mini-sermons and quoted Scripture often. Neill lived diagonally across from Ron Williamson (now a death row exoneree), and at once, Neill explained Williamson’s mental behavior and problems in his letter to Williamson’s sister Renee. During his last days on death row, Neill described his spiritual journey since his incarceration; the letter concluded, “Above all, I enjoy the sharing of love, and positive thoughts. I’m as unjudgmental as I know how to be. I believe every person has an individual right to live their lives free of harm, and prejudice. I just wish I knew more about life when I was a confused 19-year-old – the age I was, when I committed this crime.”

His appeal was denied by the U.S. Supreme Court on October 7, 2002, and he was executed by lethal injection on December 12. His final meal consisted of a double cheeseburger, fries, peach or cherry cobbler, a pint of vanilla ice cream and a large bottle of cran-grape juice. As he was on the gurney, he offered an apology to the families of Bruno, Bowles, Mullenix and Zeller, and to the Robles’, saying, “I want everyone to know I’m really sorry for what I did to you. I’m not sorry for dying here today. I’m not sorry because I’m lying here. I’m sorry for the horrible, horrible thing I did. I hope you find some comfort in that, to know Robert Johnson wasn’t in that bank. I know you think he was, but he wasn’t. Please forgive me.” As he made that statement, his voice quivered and he complained of being dizzy before asking, “Are they starting?” Then he prayed until he lost consciousness, and was pronounced dead at 6:18 p.m.  Neill was the 54th person executed in Oklahoma since the state resumed executions in 1990.