Lets take it further.
Lets say that you are a veteran returning from Afghanistan and you have PTSD or Post Traumatic Stress Disorder. You've served several tours of duty and you are now home and you want to buy a gun.
PTSD is a mental illness and many shooters in the past who have committed murder have had PTSD while 99% have not committed murder (yet).
So, now we have someone who served his country with emphasis on using a firearm and now that he's home, what should his psychiatrist say based on the vets PTSD alone?
Edit: Should a Veteran have to make the choice between getting help for his PTSD and his right to own a firearm?