From:  The American Psychological Association

"Often people who act violently have trouble controlling their feelings.  Others may have hurt them.  Some think that making people fear them through violence or threats of violence will solve their problems or earn them respect.  People who behave violently lose respect.  They find themselves isolated or disliked and they still feel angry.

If you see these signs violence is a serious possibility:

Loss of temper on a daily basis

Frequent physical fighting

Significant vandalism or property damage

Increase in use of drugs or alcohol

Increased risk-taking

Detailed plans to commit acts of violence

Announcing threats or plans for hurting others

Enjoying hurting animals

Carrying a weapon

What  You Can Do:

Be safe--Dont Go It Alone

Dont spend time with the person

If possible without putting yourself in danger, remove the person from the situation that is setting him off

Tell someone you trust, talk to an experienced professional, counselor, family member, teacher

If you are worried about being a victim of violence, get someone in authority to protect you.

Dont resort to violence to protect yourself.