They have an army protecting them. Or a society that has an armed populace. History is filled with armed groups killing unarmed, peace protestors or pacifists. One of the most extreme examples was the Hutus killing their own peace-filled Hutu people. The Hutu military threatened Hutu civilians that if they did not kill Tutsi, then they (Hutu civilians) would be shot and killed. And they did carry out their threats.
Evolution and natural selection have nothing to do with pacifists. Society does. Apples and Airplanes.
If we accept evolution and natural selection (or if we don't then fine), but if we do, then to quote Darwin ...
"I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection"
It seems reasonable to presume that pacifism did not always exist, that it did in fact come into being as a 'slight variation" and since it seems to have been preserved, then, according to Darwin, it must be useful. Of course we would have to acknowledge that any ideology, while useful to some will be deemed a threat to others, and thus your example of the Hutu.
The argument is that because pacifism exists, it must be useful and that usefulness must be a source of protection for the pacifist. No?