Mass Murderers And Injustice

I enjoy reading your post. More often than not, mass murderers tend to target particular victims to avenge perceived injustices. There are also, of course, random and indiscriminate patterns. In almost all cases, innocent bystanders get caught in the crossfire. The more random the pattern, the more likely the perceived injustice is small and insignificant. What can be done to protect society completely from mass murder?