As we learn more and more about the shooter in Arizona and of course we are now learning about all the "warning" signs.
Of course after the fact, so many things seem clear but I started thinking about my day to day.
I'm taking classes now in Philly and I gotta tell you, I've sat by people who had "off' behaviour. What do you do?
Even if you live next door to a person who is not quite as "normal" as every one else, don't you pretty much "mind" your own business.
My sister's SIL is in a psychotic state right now and there really is nothing you can do until she "does something" to be honest.

She has no insurance, she is 41yrs old, no job, and is scared to death about her mother dying and being homeless. Her concern is valid, btw.
However she is threatening my sister, her family and her own mother. She calls the police, has been thrown out of the hospital many times when her mom was in there and harassed the hospital. (My sister & BIL are caring for her in their home. They found out that SIL is nuts at that point, although the MIL knew. Long story, short version.)
You can't have someone arrested because they "might" do something.
People are free to live their lives as they see fit whether that is medicated or unmedicated.
I also have a schizophrenic MIL and a cousin who is schizophrenic.
With my MIL we demanded she take her meds otherwise she would not get to see her grandchildren. She has been doing well for 15yrs on it.
With my cousin, my aunt & uncle ingnored that he was mentally ill. Yrs after my uncle died my aunt finally found him a group home when he was about 37.
She never sees him or talks about him. He is forced to take meds in the group home, thank goodness because his schizophrenia is severe.
There is just so much that extends over into the families of people with mental illness. Some families are able to handle it better than others and some cannot handle it.