Suggest gently that his pulling the trigger was the real problem.
Exactly. If a law abiding citizen had bought the same type of gun on the same day and gone to the same shopping center, bought some chewing gum and went home, without ever taking his gun out of its holster, should he be charged with a crime?
Of course, Loughner HAD previously committed crimes that could have led to charges that would prevent him from legally buying a gun, and behaved in a manner that could have caused him to be receive a mental health evaluation and treatment or commitment. However, the vitriolic, uncivil, partisan, incompetent, Liberal, Democrat sheriff chose to ignore his behavior and allowed this menace to society to walk free and kill.
You sound like one of those unreconstructed "bitter clingers" who seem to think that people just "do things" because they "want to" — "free will".
The fault is probably in your upbringing. If there isn't therapy for that, there soon will be.
(This post brought to you as a product of my own upbringing. Void where prohibited. Not available in all areas. MSRP does not include taxes, tag, and title. Your mileage may vary. Medication should be taken with plenty of water.)