The interview with Emil Czeczko, the fugitive Polish soldier is quite something, at least the bits that are available, and those that I could find so far. Whether he is talking the truth I don’t know but he literally says that reason for his desertion were extrajudicial killings by the Polish border guards, and other security services. I translate:
We we hunting some lonely man, then we drove him to the woods, excavated a hole, and then when we were totally drunk, right in front of us, when we did not realise what is happening, the border guards shot him straight to the head. They selected soldiers, got them inebriated and then told them who to shoot.
…
I have seen two such situations. One (volunteer) was shot in the head, and the other one was placed in a row with the rest. There has never been a situation when we guided the migrants, and we did not kill them. We have always killed.
Insomniac Resurrected 