Here is a list of characteristics that are often associated with people with a high potential for violence. They don't need to display all of these signs to make them worth steering clear of.
Low Frustration Tolerance
Impulsive
Emotionally Erratic
History of Childhood Abuse
Loner
Overly Sensitive
Blackouts
Threatens Violence
Always Blames Others for Problems
Substance Abuser
Mental Health History
History of Violence
Bizarre Beliefs
Physical Defects
Pre-Occupation with Violence
History of Behavior Problems
Cruelty and Authority Issues