Liberals want proof that military service breaks people, turning normal healthy individuals to angry brooding killers ready to snap at the drop of a hat.
They believe that violence is always evil and solves nothing. It isn't logical, but logic isn't a liberal trait.
A mother using her gun to shoot the man trying to rape her daughter is using righteous violence. A soldier using his rifle to shoot the guy pointing the RPG at market shoppers is using righteous violence. A surgeon using a sharp knife to cut out a tumor is using righteous violence.
Not all doctors stay doctors, the stress of the job drives a significant number away. Also doctors can be murderers too, google "Dr. Harold Shipman" sometime.
Then again doctors kill more people every year than "psycho" veterans do. Of course the only Hollywood example of a psycho doctor that I can think of is Hannibal Lecter. You'll never drink chianti the same way again...
But there is a difference between Hannibal Lecter and the psycho veterans. Lecter's backstory is one of abuse that somehow twisted him into a cannibal. The backstory for "Taxi Driver" was simply that he was a vietnam vet. It couldn't be that the life experience he had before going to Vietnam had more to do with his behavior than a one year stint in country.
War brings out the extremes in people. The good and the bad. But that good and bad existed in those people long before they ever went to combat. That is a fact that Libs don't want to acknowledge.