If he had been knocked off in 1941 it would have saved us the cost of keeping him locked up for forty years.
Often, there are very good reasons for keeping some POWs, spies, and other foreign agents alive in captivity. In these cases, their military and political value more than outweigh any fiscal costs of keeping them incarcerated.
The same cannot be said for common criminals, who exact a far higher financial burden on society, yet with zero offsetting societal value.