I’m confused about why the gun is responsible for the Fort Hood rampage?

The gun was purchased, legally, by an officer in the United States Army. The gun didn’t walk onto the base by itself, and the gun didn’t pull its own trigger.

Responsibility lies on the person who pulled the trigger; responsibility lies with the guards at the gate who didn’t determine that the Major was armed; responsibility lies with the Army itself. Why on earth was this person promoted to Major with all the odd things being noted about him?