Potential to cause harm is not justification for anything.
If someone approaches me in the dark and starts asking questions, I will a) keep my distance, and b) answer or refuse to answer questions depending on whether I believe they are legitimate questions. If someone is asking me to justify my presence, I would probably answer "What are you doing here?" or "Do you live around here?" If someone is cornering me, and saying things like "You lost? You don't belong here." I might feel threatened, but attacking the questioner is still not okay. I may be legitimately afraid, but I am still not in imminent danger. If they physically try to prevent me from leaving, or wrestle me down, or strike me, I am justified in using force to retaliate, but even that is not legal justification for the use of deadly force, even under Stand Your Ground. Again, imminent danger of death or great bodily harm is the standard. If they are pointing a gun at me, or pull a knife, or are beating my head against the pavement, then deadly force is protected under Stand Your Ground.
Zimmerman's wounds to the face and the back of his head support the justified use of deadly force against his attacker, Trayvon Martin. I am unaware of any evidence of Martin being in imminent danger of death or great bodily harm prior to the altercation in which Zimmerman sustained injuries, but Martin apparently did not.