If I am walking along, minding my business, or worse hiding from a you, a would be assailant, and you so much as push me I am going to fight back with everything I have especially if I know you have a gun.
If someone is following me, in the dark, at night, I can pretty much presume that they do not have good intentions.
Questioning my escalation of force while justifying Zimmerman's is rich.
Again, I don't know who started it. It could very well have been initiated by Martin, If so Zimmerman's claim of self defense is justifiable. But, you don't know who started it either.