Perhaps super-lax gun laws allow problems like this to occur easier, but they certainly don't cause such problems. The cause of such problems is violent video games mental instability combined with external circumstances to push you over the edge. 
...not to be an armchair shrink or anything. 
I think you are on target here, Sandwich. I actually am a "shrink," (a Medical Family Therapist, to be exact, and I have worked in inpatient psychiatry with the toughest mental health cases) and I have worked with other cases that could have turned out just like this had the signs not been identified and had we not resisted the external pressures to discharge the patient. I had one recent case where the patient was litterally going to be released in less than 40 minutes when I was working with her and uncovered the fact that she was feeling extremely homicidal towards her mother,
who was with us in a family therapy session and who was going to be the one to drive her home upon her release from the psychiatric unit . Could we say for certain she was going to hurt her mother? No, but I wasn't willing to take the risk, and we kept the patient for a while longer until she was stablized.
When people are thinking delusionally and are under the influence of compulsive thoughts towards homicide or suicide, you have to take everything seriously. From what is being released, this guy was practically screaming "I'm unstable and dangerous!" Sadly, the mechanisms that were supposed to identify him and help him were bypassed, probably by people who felt that they meant well at the time.
The sad thing is, mental health is often considered a "luxury" service, and there is a real third-party and governmental pressure to get people out of treatment as soon as possible and as cheap as possible, sometimes regardless as to whether a real threat remains or not. While this is a tragedy, I am hoping that this helps people to reconsider the way mental health services are regarded in their place in the US health care system. Maybe, then, we will be given the resources we need to identify these cases early and prevent similar tragedies.