This Daily Beast article makes some key points:
If mental illness were the key factor in multiple gun homicides like Newtown, then other countries would regularly experience the kind of carnage visited on towns and cities in the U.S. on almost on a monthly basis. But they don’t. In Britain, an advanced study by Manchester University into “Suicide and Homicide by People with Mental Illness” has found most people who kill more than one person are neither mentally ill, nor mental health patients, As Dr. David H. Barlow, a senior expert in comparative mental health-care systems and Emeritus Professor at Boston University, told The Daily Beast, “the incidence of mental illness is quite consistent across Europe and America.” Yet the statistics for the homicide and suicide rates are much higher in the U.S. than most of the rest of Europe, with Americans 100 times more likely to die to a gun-related death than in the U.K.
Here's the link to the whole article: http://www.thedailybeast.com/...