I believe that some amount of collective responsibility exists.
Not only do I NOT believe that any "collective" responsibility exists, I especially don't believe it when the perpetrators are freaking DEAD AND GONE. It is individuals who are directly responsible for their choices, good or evil - legally and before God. And if a large group commits wrong? Each one is responsible as an individual.
I always figured I was a little crazy, but it's nice to have an expert officially cross the t's and dot the i's. Now, if I could just get an official certificate I can show to all and sundry with the words "Certified officially insane by little jeremiah".