While much of the discussion about stopping mass murder such as the recent slaughter at Sandy Hook Elementary School there is also a lot of discussion about dealing with mental illness.
Unfortunately, we can thank a fairly liberal Supreme Court for crazy people on the streets.
(M)any mentally ill people were removed from hospitals and asylums and set out on their own. That happened because of several court cases, the most critical one being a Supreme Court case in 1975 called O'Connor v. Donaldson (Wikipedia entry here). The court ruled that someone may not be incarcerated in a mental institution against their will unless they are a danger to themselves or society. To do so would be a violation of their basic civil rights to liberty and as a result many people who had been thrown into these institutions were set free.
This decision made it much harder to keep the mentally ill off the streets. The most obvious result is that a lot of the homeless are mentally ill, but another effect is that it's hard to keep potentially dangerous people who aren't obvious dangers to themselves or others incarcerated.
In the interest of preventing mass killings—and if we can't really do a whole lot about guns—should we make it easier to incarcerate people ranging from the mentally ill to disaffected goth high school boys in black greatcoats?