Yes, they should be crippled by guilt. Guilt is good -- it prevents you doing the same bad things again and gives you a perspective of what you did.
People might not live in the past, but countries carry their past with them.
The fact that the actual people that started the Vietnam war have died, doesn't mean that the country and the organisations that they worked in died with them. Those should carry the guilt forward. (Of course, instead they usually carry forward the same long term strategic planning that led to their previous attrocities).
What I mean is, countries are not discreet: they are continuous. A country has a history like a person has. Its history doesn't die and start from zero with each generation.
That's even an accepted norm of international relation even when the change inside a country is from a state to another (e.g the US after independence) etc. It's called "succession of state")
http://en.wikipedia.org/wiki/Succession_of_states
Now, of course the truth is that they are not crippled by guilt at all -- they never stopped doing crimes on humanity and on their own citizens ever. Look what they are doing to the native americans in Dakota still. Look at Zimmerman.