The question, as I see it, isn't not so much what is right, or whether even what I/you personally would have done in their stead (as you might have noticed from other threads, I'm more the soldier-type, and as far as risking being stabbed myself on someone else's behalf goes, also proven my willingness to intervene in real life), but rather what should or shouldn't we punish.
Is inaction during crimes a punishable offence?
Is following an orders under death-threat a punishable offence?
Is saving yourself and your family over strangers a punishable offence?
Is choosing people you know over helping strangers a crime that warrants punishment over half a century after the fact?
If inaction during crimes is a punishable offence, then what should we do with all the people who witnessed a brutal crime at current times, but did not want to be involved, even if "involved" meant going home and calling the police from the safety of being behind a heavy metal door, and later saying "Yes, this is who I saw"? In my eyes, a person who watches a a child being raped and scurries off without telling anyone is at least as guilty as this man, but we don't exactly go around putting individuals like that on trial, despite the fact that those people are here and now, and are by no means not under the same kind of threat from the system. Where do we draw the lines?
He did not decide to kill anyone out of his free will, or give the orders to. He did not even personally conduct the executions. He was a guard. Over half a century ago. Aside of vindictive satisfaction, there is nothing to gain from convicting an old regretful man in a wheelchair. He's in no position to go and harm someone else anymore, and he wasn't an active participant in the harming anyway. He was pretty much a bystander.