I'm all for considering externalities, your examples are terrible, especially Trayvon. It's arguable whether he did anything wrong, and it's also quite possible he wasn't high at the time[0]. Even if he was high and was aggressive once Zimmerman started following him, there are more important factors in the disturbance than how Trayvon acted.

0 - http://en.wikipedia.org/wiki/Shooting_of_Trayvon_Martin#Coun...