Wouldn’t that depend on Martins suspicious behavior? What if Zimmerman didn’t follow him, Martin realizes nobody is around, breaks into a house/car and gets away? Zimmerman was doing the right thing by keeping an eye on him, from what I can tell he messed up by approaching him. It seems, if the story is true that we know, Martin messed up if he started the punching.
It’s all that matters. If someone pulls a weapon on you and you feel your life is in danger you have the right to take matters into your own hands at that point. Whether its a knife or a gun. But you don’t have the right to throw a punch just because the other guy looks like he might throw one first.
There are a lot of situations where someone may pull a knife or gun and the smart thing to do is to let them have what they want and go. But if they do so out of anger and you believe they have every intent to use it on you then by all means, you might want to find a way to defend yourself and retaliate if possible.