For one thing, stalking is absolutely an aggressive act. Further, the presence of the gun indicates that GZ almost certainly wasn't in fear of anything, since he was capable of responding with deadly force - of course, the fact that he followed Martin to begin with also indicates his lack of fear.
If you are being followed by an unknown individual (or even known, if you know the person to be dangerous) is FRIGHTENING. I'm baffled as to how certain people don't think it wouldn't be. Frightening enough to flee, which Martin obviously did. Zimmerman even admits that. Perhaps the person who knowingly has the ability to respond with deadly force and obviously isn't afraid for his life is the aggressor in this situation. That is what makes the most sense.
What gives a person the right to respond is being confronted, which is exactly what happened when GZ confronted TM. TM was also within his rights to demand to know why GZ was following him, which he did. As to what GZ did at that point, we don't know. Given his lack of fear of Martin, and his ability to respond with deadly force against what was obviously a kid, Zimmerman, imo, likely tried to detain Martin. Martin for his part, likely didn't feel like being detained by some random stranger. Fight ensues. That's on Zimmerman, not on Martin. If you get your tail kicked after starting fight, it's your own fault.