From much reading about WWII I know that there were many instances of war crimes and atrocities committed by the Wehrmacht. But I haven't been able to find any sources on whether it was a very large proportion of Wehrmacht troops involved or if it was more isolated. I was under the impression that most of the Wehrmacht was conscripted, and so didn't assume they were all the same rabid, genocidal Nazis that the SS and the SA were. Am I wrong?
I have previously wrote answers to similar questions here, here, and here.
The gist of it is that it is really hard to say. While the whole upper echelon of the Wehrmacht as well as the Wehrmacht as institution was deeply involved in war crimes, when it comes to the numbers of individual soldiers, it becomes incredibly hard to say. Not only because of the numbers involved and the research still needing to be done but also because of the questions of what constitutes complicity in war crimes. The Wehrmacht frequently did guard duty for Einsatzgruppen executions. Are the people guarding the victims to be shot complicit in said war crime? Is transporting Jews from Ghettos to Auschwitz something that makes one an accomplice?
In light of this, it is very hard to estimate. The overwhelming majority of Wehrmacht soldiers knew of the crimes that were committed and it is a fair estimate that almost every unit in the USSR, Serbia, Poland, and Greece was at least witness to some sort of war crime.
As I mentioned, there is still research to be done in this area and it currently is underway using the eve's dropping protocols from American and British POW camps but the most useful metric so far comes from Christopher Browning in his book Ordinary Men where he found that in a Police Bataillion stationed in Poland 20% of the members were willing killers, 20% refused to go along with it, and 60% were pressured into taking part.