I don't see the consistency problem here -- membership in the Nazi party alone does not make anyone a war criminal. But anyone involved with the concentration/extermination camps is a different matter entirely.
Depends whether the "involvement" was truly voluntary or not. We're going to be facing this issue in a big way whenever we finally get rid of "Dear Leader", and the new Iraqi government is already facing it. How guilty is someone who "follows orders" knowing that the alternative is to have his parents/spouse/children raped and tortured and then executed? How many of us could really stand up to such orders under those circumstances?
I don't believe the Nazi tactics for gaining compliance were that extreme, but I'd really like to know what the consequences were for low-level Nazi soldiers who refused to carry out their assigned duties as concentration camp guards, before I support hounding these old geezers 'til the day the die.