This does not even appear to be an over-zealous citizen making an arrest after witnessing a crime. It APPEARS only one crime was committed. Even if the kid pulled a knife on the street to protect himself against another armed man, the deceased appears to be a murder victim.
Blind Men and Elephant
http://wordinfo.info/unit/1?letter=B&spage=3
Moral:
So oft in theologic wars,
The disputants, I ween,
Rail on in utter ignorance
Of what each other mean,
And prate about an Elephant
Not one of them has seen!
God Bless and help us all!
So what if Zimmerman approached Martin with his weapon concealed and simply asked whether Martin lived nearby and in response to this Martin rushed Zimmerman and started punching him in the face?
If that was the case, would it be murder for Zimmerman to draw his weapon and defend himself?