Beevor in Stalingrad quotes Helmuth Groscruth, an Anti-Nazi officer who states his disgust and outrage at the atrocities being perpetuated by his service and government in the USSR. Unfortunately I haven't been able to find the actual letter he wrote to his wife (not that I've looked particularly long and hard), but perhaps this is a good starting point for you?

Here's a screencapture of the excerpt I'm referring to. I don't doubt that if you dig deep enough you can find a suitable translation of Helmuth Groscruth's correspondences. I know a single name isn't very helpful but its better than nothing; the sad fact is, most of the Germans were quite indifferent to the plight of the invaded even if they weren't supportive of the war. You will have to go out of your way to find men - especially Officers, openly critical or unsupportive of the regime while they are still winning.