The mysterious killing of one of France's top secret service agents in February could have some connection with the mass expulsion today of Soviet diplomats and other officials from this country.

When the body of Lt. Col. Bernard Nut was discovered on a winding mountain road north of the southern port of Nice with his Smith and Wesson .357 Magnum near his head, his colleagues in the French intelligence community insisted at first that he had committed suicide. This theory was soon discarded, however, and this week word leaked out from investigators that the 47-year-old agent had been murdered, probably by a close friend or associate.

The most tantalizing hypothesis raised so far for Nut's murder is that he was killed by a double agent because he knew too much about Soviet Bloc espionage operations in Italy and southern France. His work for the Alpine division of France's DGSE foreign intelligence agency took him frequently to Italy where he was reportedly involved in the unmasking earlier this year of a KGB colonel, Viktor Pronin, who was caught with microfilms of western defense plans.

Much remains mysterious about the affair, and it is impossible to confirm reports in the French press that Nut also may have been involved in assisting the Italian secret service in investigating the so-called "Bulgarian connection" in the attempted assassination of Pope John Paul II in 1981.

French intelligence sources have said that Nut was "on a mission," at the time of his death on Feb. 15, but have refused to specify what it was. Nut told colleagues that he had a meeting with a contact.

An autopsy on Nut showed that he had consumed food and alcohol less than three hours before his death, and one of the hypotheses on which investigators are now working is that he dined with his murderer. Forensic evidence suggests that the killer acted very quickly and did not take careful aim. It also proves conclusively that Nut could not have shot himself.