The exact same thing that's wrong with every country where innocent people can be killed by some disaffected person using guns, knives, or any other weapon.
Stop focusing on one kind of weapon and read the news more broadly. School massacres happen without guns. Dead joggers happen without guns. I heard anti-knife public service announcements on the radio in the UK the year before I came to China. From the tone of the announcements, you would have thought they were talking about grenade launchers.
I believe the source of the real problem is 2-fold. First, the sense of personal responsibility seems to be waning faster and faster in many supposedly developed countries. Second, the concept of "mainstreaming" the mentally ill allows those who really need treatment (or possibly involuntary committal) to slip under the radar, often never even getting a formal diagnosis of mental illness until they are working with a defense attorney.