Holocaust Justice

Rose
Should Nazis be held accountable for their actions?  I'm not sure if this is relevant anymore or not (maybe everybody involved in the Holocaust has passed, idk) but I'd still like to know. 
( I got to thinking after reading this book titled the Storyteller.  It is mainly about tracking down Nazis and putting them on trail, in the year of 2012).
During the holocaust, Nazis were given the O.K. to do what they did to Jews and anyone else they deemed unfit of living.  It was legal at the time.   After the Holocaust was over, that wasn't the case of course.  What I'm trying to find out is:  should you punish someone for a crime they committed, even if the "crime" was legal at time?
That would be like legalizing every drug ever and after placing a ban on those drugs maybe years later, you arrested the people that used the drugs before they were banned. Can you go back in time to punish people for something that wasn't considered a crime in the past but is considered a crime in the present? 
EDIT: Maybe legal isn't the right word. But their leader told them to do it. And like someone said, there were some Nazis that didn't want to be involved but were because they  wanted to survive and/or protect their families.