WITH their plumed helmets and medieval-style stripey doublet and hose, the Swiss Guards, who stand stiffly at the entrances of the Vatican, look uncannily like toy soldiers newly escaped from the pages of a children's fairy story. But the gruesome triple killing which rocked the Holy See on Monday bears all the hallmarks of a thoroughly modern murder.

Alois Estermann had waited seven long months for the news he had heard that morning. The 43-year old devout Catholic was told at last he was to be the new Captain Commandant of the Swiss Guard, the world's smallest army.

Nobody knows for sure why later that day 23-year old Cedric Tornay, a vice-corporal in the same force, went to the Vatican apartment Estermann shared with his Venezuelan wife Gladys and shot them before turning the gun on himself.

One theory is that the younger man went berserk after being reprimanded by Estermann for staying out all night. Many of the elite corps are said to resent the midnight curfew which hampers their Roman nightlife. Tornay also had a chip on his shoulder about being under-appreciated.

These are the only violent killings in living memory to shatter the sleepy, cloistered atmosphere of the Vatican. The last was in 1848 when Pope Pius IX's prime minister, Count Pellegrino Rossi, was assassinated during the political unrest surrounding the unification of Italy.

Nevertheless it is the third incident this year to rock the Vatican. In February, a gay man set fire to himself in St Peter's Square, apparently in protest at the perceived homophobia of the Church. The previous month, one of the Pope's tail-coated personal equerries was found battered to death in his flat near the Vatican. He was wearing only underwear and there was a homosexual pornographic movie in the video player.

While nobody is seeking to link these tragedies, there is a recognition that at a time when the Roman Catholic Church is under fire on a number of fronts and many adherents are drifting away, such incidents further tarnish the institution's image. The mystery surrounding the crash in 1982 of the Vatican's bank, the Banco Ambrosiano, and the subsequent discovery of the body of its president, Roberto Calvi, hanging under London's Blackfriars Bridge, has never been fully resolved.

Yesterday there were scenes of shock and grief among the close-knit group of 100 or so Swiss Guards who volunteer to lay down their lives for the Holy Father.

Lousy pay (#535 a month) and long hours standing to attention holding a seven-foot halberd (a cross between a spear and a battle axe) have led to recent recruitment problems. In addition, applicants have to be Swiss, Roman Catholic, under 30, of good social standing and ''irreproachable morals'', and they must be at least 5ft 9ins tall.

For sentry duty outside the entrances to the Vatican they wear their distinctive doublet and hose in vertical red, yellow and blue stripes, reputedly designed by the Renaissance artist Michelangelo. Swiss mercenaries formed the backbones of many Renaissance armies, including that of Pope Julius II, who struck an agreement with two Swiss cantons to provide a permanent army.

Today's Swiss Guards resent the insinuation that they are little more than Rome's answer to Beefeaters. Few members of the public realise that the guards have another uniform: a grey suit with a distinctive armpit bulge.

That's what Estermann was wearing in 1981 when, as a new recruit to the Papal guard, he jumped onto the moving ''popemobile'' in St Peter's Square seconds after Turkish gunman Ali Agca shot and nearly killed John Paul II.

Rennie McOwan, author and Scottish correspondent to the Roman Catholic newspaper, The Tablet, has been stopped and questioned by Swiss Guards during a trip to the Vatican. ''Security used to be rather lax but it has tightened up a lot since 1981. The Swiss Guard have both a symbolic function in their colourful uniforms and a practical one because, in a very real sense, they are security guards,'' he says.

Yesterday McOwan, who has criticised the Vatican for being unnecessarily secretive, praised the administration for speedily organising a press conference and answering questions as openly as possible.

''This is a step in the right direction. It's the sort of thing that could happen in any institution: among the staff of the royal family; at 10 Downing Street; or in the upper echelons of the Conservative Party.''

Catholics resent the attention focused on such an incident at a time when the Church is wrestling with a series of major problems.

Says McOwan:''It would be a mistake for commentators to link such events with the major theological debates going on within Roman Catholicism about issues such as priestly celibacy, contraception, the role of laity and the ordination of women.''