Why does it distress Paul to watch the Russian prisoners?

From the text:

It is strange to see these enemies of ours so close up. They have faces that make one think-honest peasant faces, broad foreheads, broad noses, broad mouths, broad hands, and thick hair. They ought to be put to threshing, reaping, and apple picking. They look just as kindly as our own peasants in Friesland.

It is distressing to watch their movements, to see them begging for something to eat. They are all rather feeble, for they only get enough nourishment to keep them from starving. Ourselves we have not had sufficient to eat for long enough. They have dysentery; furtively many of them display the blood-stained tails of their shirts. Their backs, their necks are bent, their knees sag, their heads droop as they stretch out their hands and beg in the few words of German that they know--beg with those soft, deep, musical voices, that are like warm stoves and cosy rooms at home.

Some men there are who give them a kick, so that they fall over;--but those are not many. The majority do nothing to them, just ignore them. Occasionally, when they are too grovelling, it makes a man mad and then he kicks them. If only they would not look at one so--What great misery can be in two such small spots, no bigger than a man's thumb--in their eyes!