About the psychiatrist being able to see that he was planning this... people say things like this all the time, that doesn't necessarily mean they'll do it. The psychiatrist might even have reported it to the cops, chances are he would have been interrogated and let go anyway.
And don't get this wrong, psychiatry is like a fad here. So many people go into those offices and say whatever. The psychiatrist really can't tell whether or not you'll do something. even if you say it, because the extremity of words is so common here (especially in psychiatrical offices) that it is barely ever taken seriously. Most times it's just regarded as "honest venting."
I was more concerned about the whole part where someone can actually buy four weapons like those ones - AND on top of that have a psychiatrist who picks up those tendencies. The psychiatrist bit would be the last step, the last check, especially since it's supposed to be confidential. The selling of those weapons would be the first flag - I'd say it shouldn't even happen. If anyone is concerned about that, then they should speak up, make a group, etc.
Wherever he bought them probably had a bunch of people come and do the same without hearing about a mass murder from 4 months later.
The fourth one to sell him his heavy weapon sold a heavy weapon to someone who already had 3. Why would that gun seller think it was okay ? Because he didn't know Holmes had 3 like that already ? So why did he not know, then ?
Psychos who are accounted for wouldn't be allowed to buy weapons. Holmes's psychiatric treatment apparently wasn't that formal(?), so a background check was unlikely to bring that up.
Background check might not have picked up that he was a psycho, but as I was saying to len, it certainly should have picked up that he had 3 heavy weapons already. Even 2. Hell, that he already had even one assault rifle[/spolier]