Mass killings are rare in other developed nations as they all have restrictive guns laws.   We will always have citizens with mental health problems as this fellow had, and we will always have social and political problems that can cause these unstable people to get angry and seek revenge.   

We cannot eliminate the root causes, but we can reduce the ability of such people to kill by restricting access to guns.   Unless we are willing to do this, these mass killings will continue.   No one is safe, even those who always carry a gun.    These victims were armed and trained police officers.  If one angry and disturbed man can kill and wound so many of them, then no one is safe.  

We do not have to live like this.  We just need to take a stand against the gun manufacturers and their lobbying organization, the NRA.