The Table at the bottom of this article shows the US mass murders for the last two years which took the lives of 93 people and injured another 123.
There are three important points that can be gleaned from the table. First, probably none would have passed a Canadian background check or been able to pass the gun safety course. Even if the killers had obtained guns it wouldn't have been the assault rifles that are very difficult to obtain under Canadian law. In addition, Canada has a 26 day waiting period. Some mass murders only obtained their guns within days of their murders.
Second, contrary to the mainstream media, most mass murders are reported delusional before their attacks and many had prior diagnoses of paranoid schizophrenia.
Third, some mass murderers would have received inpatient treatment for their schizophrenia prior to their shooting in Canada or other public health system countries. Furthermore, in this country the mentally ill are frequently denied medical care because they were thrown out of school, lost a job or went bankrupt, and because President Reagan destroyed the national health care inpatient system for the mentally ill in this country. A system that was strengthened under Jimmy Carter. Even those who asked for treatment were not hospitalized as they would have been in other countries. Reagan's policies forced many mentally ill patients out on to the street where they couldn't function. Therefore, many ended up in prisons.
Gun related homicides in Canada are about 1/6 as common as they are in the U.S.
UPDATE: Under Canadian law the mother would not have been able to purchase any of the guns used in the CT shootings. It's still unclear whether she did or not. A court ordered her to take parenting classes when she divorced her husband several years ago.
Also the mother has been described as believing the world was going to end and a survivalist who was overprotective. Her mother was described by neighbors as a paranoid "prepper" which means she was preparing for the breakdown of organized society and that's why she had stockpiled the assault weapons. She also insisted on never leaving Adam alone and that no one come inside her house. Neighbors said boy was prone to serious temper tantrums at age 10. His older brother hadn't seen Adam in 2 years. There are continued reports that he has Autism but there are no reports that he received medical treatment or an official diagnosis. He was home schooled in his last few years of high school.
One commentor argued that the family new of the mental health issues and addressed them but so far there is no evidence of that and more evidence of a paranoid and isolated existence.
Although most mass murders have paranoid schizophrenia, a smaller group can be classified domestic terrorists. Given mother's extremist views, that's another possibility. An example of this sort of person would be Luke Woodham who also was mentally retarded but came under the influence of cultists or Jeffrey James Weise who was suicidal and had Nazi sympathies. However, Kip Kinkel was also initially viewed this way but it was later determined that he had paranoid schizophrenia as well.