no, I don't think he should be punished at all if he merely served as a guard in a camp where lots of people were killed. He is an old man with a family here in America and he should not have his citizenship stripped at age 82. Look at it from Demjanjuk's point of view. If he failed to work for the army, then he may have been killed for it. He was born in Ukraine where 6 or 8 million were killed in the 1930's by a similar actions as to the holocaust. If he is 'guilty' he was likely just trying to survive. He knew well that failure to serve the authorities could cause death, that was the environment he knew. For this we should only forgive at this point.
Don't you all believe in forgiving a fellow who was just trying to make his way through this world and in a situation where it will do nobody any good to prosecute?