THOSE INTEREST was served by the assassination of Sen. Benigno Aquino Jr. as he stepped out of the airliner that had brought him home to the Philippines? Like all the other questions, that one has no immediate answer. For the legitimate opposition to the Marcos regime, it is an irreparable loss. There is no other figure of Mr. Aquino's stature to step forward. But the event also harms President Marcos, his family and his government by drawing attention to the less appealing aspects of their rule--their reliance on military force, their greed and their increasingly vigorous suppression of dissent in recent months. President Marcos appeared on television yesterday to assert his innocence and to suggest that the killing was the work of communist guerrillas. But weakening the legitimate opposition does not necessarily help the other type.

Whoever organized it, the operation was a model of its kind. Mr. Aquino had hardly stepped onto Philippine soil when he was very efficiently executed with a shot to the back of his head, from very short range. His murderer was then immediately shot dead by the security men who were there in great numbers. There won't be any interrogation. The government says that the assailant still has not been identified. The imagination suggests any number of intricate double and triple crosses that might have been played out in these few seconds. How did the gunman expect to get out, and what might have been promised him?

The strangest thing about this affair is the explicit sense of personal danger to Mr. Aquino that had hung over him from the time that he decided to return to the Philippines. He had told friends that he had been warned by no less a personage than Imelda Marcos, the president's wife, of plots to assassinate him. He returned home with an evident sense of foreboding; when he left the aircraft, he was wearing a bullet-proof vest. The whole sequence moved too smoothly to its conclusion. It was not only, as everyone has said, reprehensible. In view of details like the dozens of security men surrounding him, the bullet-proof vest and the welcoming crowd of several thousand that had been allowed to gather at the airport, it was also exceedingly strange.

Perhaps the government had nothing to do with it. That's possible. But the government certainly bears the burden of demonstrating as much. If the Marcos government has any concern for its reputation, it will pursue the case with more than normal energy. President Reagan is scheduled to visit Manila in November, as part of his trip to Asia. It will be a more pleasant trip--and a more defensible one--if, by then, President Marcos has managed to provide answers to some of the questions that now surround this crime.