"He complained to his colleagues that he was not included in the list," Navarro-Valls told reporters. The guardsman also was outraged over a recent letter of reprimand from the commander, Col. Alois Estermann, for staying out all night.
"It was a fit of madness in a person with very peculiar psychological characteristics," Navarro-Valls said.
The three-year member of the corps, 23-year-old Cedrich Tornay, gave a letter to his family shortly before Monday's 9 p.m. killings, the spokesman said. The contents of his letter were not released.
CBS News correspondent Allen Pizzey reports that officers will autopsy the three bodies Tuesday, opening an inquiry by the Vatican's only magistrate into the first killing inside Vatican walls in 150 years.
The funeral Mass for Estermann, 43, and his Venezuelan-born wife, Gladys Meza Romero, 49, will be celebrated in St. Peter's Basilica on Wednesday, Navarro-Valls said. Cardinal Angelo Sodano, the Vatican secretary of state, will celebrate the funeral Mass. It was not certain where the couple would be buried.
Wednesday's swearing-in ceremony for 40 recruits to the world's smallest army was cancelled. A flag on the Swiss Guards barracks flew at half-staff.
The commander, in civilian clothes, was at Pope John Paul II's side during 30 foreign trips, Pizzey reports. He says Estermann distinguished himself in 1981 when he flung his body across the pope to protect him when the pontiff was shot in an assassination attempt in St. Peter's Square.
The killings came on the day Estermann had reached the pinnacle of his career: Hours earlier, John Paul appointed him commander of the 100-member corps founded in 1506.
Navarro-Valls said a neighbor heard noises Monday night and went to investigate. In a small room near the entranceway to the couple's Vatican apartment, "she found the three bodies laid out," he said.
Underneath Tornay's body was his semi-automatic service revolver, the spokesman said. Only one bullet was inside, and five shots were fired.
The pope, "who appreciated (Estermann) particularly," was "visibly sad" when told the news, Navarro-Valls said. The pope described Estermann as having "extraordinarily humane, professional, and I would say spiritual qualities."
Raymond Flynn, former U.S. ambassador to the Vatican, told CBS This Morning that Estermann was "a very professional military, career officer, who I thought was very dedicated to his profession, very committed to the Holy Father and to his job as the leader of the Swiss Guard."
Flynn, who knew both Estermann and the pope personally, said John Paul has "a special affection for people ho work for him, particularly people who are loyal to him and to the Catholic Church."
Ms. Meza Romero worked at the Venezuelan Embassy to the Holy See. The couple had no children.
The bodies of Estermann, his wife and the guardsman were found just inside the St. Anne Gate, a main entrance to the Vatican. The couple's apartment is several hundreds yards from John Paul's residence.
Outside the block-long, three-story, orange building, tourists and curious Romans gathered today. Grim-faced Swiss Guards, wearing blue tunics and black berets, stood inside the gate.
The killings sent shock waves through Switzerland, which for nearly 500 years has been supplying young Roman Catholic men ready to lay down their lives for the pope.
Swiss President Flavio Cotti sent a letter to John Paul expressing the "sincere sympathy" of the Swiss government and people.
Estermann was one of the few non-noblemen to head the Swiss Guards. He joined the Swiss Guards in 1980 after four years as an officer in Switzerland's army.
The top job had been open since the previous commander retired in October. In an interview with the Rome daily La Repubblica hours before the shootings, Estermann said he saw "nothing strange" in the delayed appointment and spoke of the "enormous responsibility" of the job.
His parents arrived in Rome on Monday for the swearing-in ceremony on Wednesday but never saw Estermann alive, his youngest brother, Robert, told The Associated Press.