From all this we may learn that there are two races of men in this world, but only these two--the "race" of the decent man and the "race" of the indecent man. Both are found everywhere; they penetrate into all groups of society. No group consists entirely of decent or indecent people. In this sense, no group is of "pure race"--and therefore one occasionally found a decent fellow among the camp guards.
The “this” in “From all this…” is the author’s explanation of the grace sometimes seen from camp guards and officers in the concentration camps he was attached to.
While decency is left up to interpretation, a lot can be taken away from the paragraph.
While the “pure race” is most likely in reference to Hitler’s ideology, this paragraph is very applicable to our current world. If men were defined by their decency in today’s world, we’d all be better off.