So what? What if police did know he had three legally purchased guns and ammo? How would have that changed anything. There was no crime.
Anyone who wonders why cops didn't do more doesn't understand police. When police deal with an individual, the bottom line is only two things matter: 1) Has the person committed a felony crime? (A few states, like New York, but I don't know how many others, allow police to arrest for not-witnessed lesser crimes as well) 2) Is the person a threat to themselves or others?
Certainly owning your legal and constitutionally protected firearms doesn't qualify as a crime, so the latter issue is more relevant here. Far be it from me or any cop to say this guy was going to shoot a bunch of people. If you can remain calm and hold a half reasonable conversation with police, congrats: by police standards, you're officially sane enough! That's the way it works. (The standards are quite a bit stricter in domestic situations.)
So when cops have to judge someone's sanity, and I say this out of experience, all they can do is look for obvious signs of crazy. I'm not talking about zany, eccentric, or senile. I mean even walking around in your underwear because you say snakes were crawling up your legs probably does not qualify (cause that's a sign of a bad drug trip). By crazy I'm talking about loopy tin-foil hat wearing. I'm talking actively delusional. I'm talking no-awareness-of-reality insane. Cops look for crazy; cops look for insane. But crazy and insane aren't medical terms found in the DSM-V.
There's the problem: cops aren't shrinks; cops are not medical doctors. It is not and it should be the police officer's job to diagnose mental illness. If you haven't committed a crime and you're not clearly a threat to yourself or others, police shouldn't be able to detain and involuntarily commit you to the funny farm.
Take this case I handled during field training:
Dealt with the same mental patient in a high rise that I dealt with one or two days ago. Swearing, exposing himself, thinking the whole building was his, just being a big crazy problem. Obviously, he was a horrible person to have around. He also couldn't remember that we was in jail the past week and that he wasn't on his medication. The building management wanted him out with justification. Eventually, his mother and a "friend" talked him into going to the hospital. (The friend, an older black guy who lived in the building, was not actually a friend, but at least he was a good and caring man.)What would have been the right thing if he hadn't gone voluntarily? Cops can't make somebody take their meds. Plan B could have been to provoke him into threatening us, thus giving us a reason to take him against his will. Otherwise, his mother would have had to go through a lengthy civil process to get him committed. Or we just leave him there till he does hurt somebody. He needs help, but, as is often the case, help police cannot give.
Cops do not like mental cases and generally don't handle them well. Though admittedly some cops handle them much better than others. In certain situations police need to focus on more goal-oriented tactics -- like what is the best way to get this person to do exactly what I want him or her to do -- rather than demanding deference to police authority as a starting point. Some of this could be taught with better training, but police will never be great handlers of the mentally ill. The power of police is to detain people; the tools of police can kill people. Neither is right for the job.
Luckily, there's actually an easy solution: psychiatrists and mental health professionals. Doctors on call with judgement and the power of involuntary detention. Of course it would cost some money upfront, but in the grand scheme it saves because their prisons don't house their mentally ill. When I did my police research in Amsterdam, there was this white car with some writing on the side and a yellow mars light on top. They were the shrink squad. We, as police, didn't deal have to deal too much with these professionals because, get this, they dealt with the crazies and we dealt with the criminals. Imagine that. Separate groups. Sometimes the two worlds would overlap, but not that often. Yes, this is another un-American socialist European concept: have a system to deal with the mentally ill. Now that is crazy.