IS THERE in Africa, or anywhere else in the Third World for that matter, any military man who cannot find noble rationalization for a coup? The amount of self-justifying rhetoric lying around barracks shelves seems virtually unbounded.In Liberia over the weekend, for instance, the elected president, William R. Tolbert, 66, was dislodged -- and murdered -- in a coup led by Master Sgt. Samuel Doe, 28, who announced that Mr. Tolbert had been guilty of "incomparable corruption" and had neglected the masses. The sergeant had no alternative, he said, but to overthrown him.

Really? Mr. Tolbert, a smooth old hand who represented Liberia's elite (descended of former American slaves), did indeed preside over a fair amount of corruption. Under his and his class's rule, an awful lot of people stayed poor. He ran a paternalistic show and in the last few months he had laid on the police with a heavy hand. His True Whigs, furthermore, had won every election for more than a century -- not a record promising an early peaceable transfer of power.

Yet Mr. Tolbert was not your obvious candidate for corrupt and bloodthirsty dictator of the year. The "rice riots" of a year ago had stirred him deeply. He had sanctioned Liberia's first registered opposition party, opened his government and extended the franchise to Liberians without American roots, promised to serve only one term, endorsed a more populist conception of development, and so on. Just two months ago the State Department gave Mr. Tolbert a glowing human-rights report -- by contrast, Sgt. Doe has started out, it is reported, by summarily executing Mr. Tolbert and perhaps also his son. The classic dilemma of reform was evidently being played out: change instigated from the inside did not satisfy critics on the outside. It emboldened them.

The American roots of Liberia's elite have inclined it to accept and profit from a special relationship with the United States. On its part, the Carter administration, like its predecessors, has been pleased to cultivate this rather old-fashioned African country on which it could count for a friendly reception for American values, economic links and views. It would be surprising if the new leaders in Monrovia did not want to show they were reviewing the coziness of this tie. As they do, they will no doubt say more harsh things about William Tolbert to justify ousting and killing him. They should rule at least as well as he.