how common was psychological trauma suffered by WWII Veterans from the war?

[.^.]

and which percentage of soldiers had actually KILLED another human being? in which quantities and how most common ways?

also, which other things they saw that caused psychological trauma after the war and how compared to Vietnam veterans? and why Vietnam was said to be WORSE in this case? (how is this true really? )

please explain how long and how long after the war many began to show signs of psychological problems and how they sought help?

also, which sides saw the WORSE of WWII and why? (germans, americans, British, or Russians or Japanese?) how compared?

please explain what you can why so many like to remind themselves of this war like it was one of the best times of their life? (was it for many? please explain reasons)

thanks for your answers!