It is indeed self-defense if Trayvon was a threat beforehand. Assault and attempting to cease a weapon warrants a gun shot. The threat had to be eliminated.
The threat could have been eliminated by not chasing. That whole chase thing is what makes Zimmerman guilty. He chased someone who he had no previous interaction with. Trayvon did not steal or injure from Zimmerman to warrant being chased.
If Zimmerman is not guilty, then it is ok to chase anyone in your neighborhood out. Even people who live there. Who did nothing.
Do you want someone following you, chasing you around your own neighborhood?