Violent employee had killed other co-worker before

Christopher Yong, 39, was charged with aggravated assault, dangerous driving, failing to remain at the scene of an accident and breach of probation after he hit his supervisor with a work van and then sped away. The September 17th assault occurred at a document shredding company in Mississauga, Canada after the victim/supervisor told Yong to retrieve some paper work and the employee became enraged.
The violation of probation was for a 2002 workplace violence incident in which Yong fatally stabbed an employee that he was supervising at another business. He was charged with second degree murder but only served 18 months after receiving a manslaughter conviction. As this occurred in the same town as the last incident, it is likely that his current employers knew of the previous homicide.
Perhaps the question is not whether background checks should be done but what do you do when you are aware of previous, criminally violent behavior. Comments?