Actually, one may justifiably use force if you reasonably believe that such conduct is necessary to defend himself or herself or another against the other’s imminent use of unlawful force. Zimmerman's behavior of following Trayvon and failing to identify could easily have made Trayvon reasonably believe that use of force against him was imminent.
As for the bad names, you are just using a strawman. For the umpteenth time, I am not saying it justifies the use of force in response. I am saying it provokes and that a reasonable person should expect that it may provoke a response. To then shoot to kill, without exhausting efforts to retreat, and claim self defense seems too easy of a way to get away with murder.