Emotionally instabile people . There is no mechanism to lock them up. So we have to use our own judgement to protect ourselves. The wife of the murderer should probably have known by his previous behavior she is in danger and carried a gun all the time. Also good to keep in mind:  In 2013, a black was six times more likely than a non-black to commit murder.  https://www.amren.com/archives/reports/the-color-of-crime-2016-revised-edition/  just yesterday gym owners/supervisors were killed by a black employee.