Regardless of whether or not someone has a confirmed diagnosis, I would say that there should be ready and easy access to some sort of mental health or crisis intervention services, although such services are badly overloaded already. We have suicide hotlines and other numbers for people to call, whether it's drug problems, gambling addiction - things like that. But if someone feels anger at the world and wants to kill someone, who do they call for help? Where do they go? Even if they're not necessarily mentally ill in the clinical sense, they're obviously people who could use some sort of help from somewhere.
I still remember the Loughner case after the Gabrielle Giffords shooting. The reports indicated that he tried to get mental health services through the local community college, but apparently he never followed up or they blew him off or he just fell through the cracks. But after he did the killings, he was examined by multiple psychiatrists. They flew him out to some facility in Missouri, and they examined him, treated him, at least to make sane enough to stand trial, where he pleaded guilty. I found myself thinking, why couldn't they have seen him and treated him before he committed his crime?