1. Following is not confronting. As captain of the neighborhood watch, he not only had a right, but a responsibility to at least observe any suspicious activity in the neighborhood
2. As I said earlier, he has admitted following, losing the kid and then heading back to his truck. He is on recording as saying he's heading back to his truck. "I lost him".
3. If he lost him, how did he suddenly find him on his way back to his truck?
4. There were no specific instructions to stop following him. There was a suggestion that "We don't need you to do that." with respect to following Martin, to which he responds "okay". At which point, see #2 above.
5. Despite all of this, even if he was following Martin, the second Martin punches him, the aggressor is Martin. And if Zimmerman ends up on the bottom getting ground & pounded by Martin, he is within Florida (and almost every state in the US) law to defend himself, lethally if necessary.
All of this doesn't take into account whether it was morally objectionable for him to shoot and kill this kid. You can hold something to be morally objectionable and still have it legal. Should he have shot and killed the kid? No. Did he have a right to? Possibly.