1. The behvaior poses a risk to either themself/others.
2. The person exhibiting the behavior is distressed by it.
3. It is statisically deviant.
Also, to a lesser extent, societal norms come into play in defining what is abnormal, but the risk of using this as a criterion increases the risk of stigmatizing.
2. Abnormal - The definition of brutal death metal.
Murders, Death, War, Homophobia