You could make the argument that is(or was) the traditional method of the Muslims, and especially the Arabs. Until Saladin rose to power in the 12th Century and instated a more "ecumenical" rule after the 3rd Crusade, the Arabs - as a matter of course - would just kill every man, woman and child they came across when attempting to re-take Jerusalem. The Saracens exemplified this kind of barbaric and unforgiving behavior.
After Saladin's death, things returned to "normal" for the Arabs - back to their more barbaric ways. The Moors were a rather unsavory lot as well. Sure, the Nazi's get all the bad press, but as far as barbarism goes, they're in plentiful company, mostly with Muslims.
This required a large and productive class of non-Muslims, for which massacres would have been counter-productive. The first and probably second generation were completely incapable of maintaining an advanced civilization, yet such was maintained, indicating most of the population was not only not killed, but also were not severely demoralized by massive atrocities.
There is excellent evidence that the subject peoples of the former Roman provinces of Syria, Egypt and North Africa saw the Muslims as the lesser of two evils as compared to their previous rulers, which of course would not have been the case had they been killing everybody. While we have less evidence, it is likely a similar situation obtained in the former Persian Empire.
The first century after Mohammed's death largely consists of the reaction to the financial crisis produced when the subjects began converting to Islam in large numbers in order to avoid taxation.