As knowing goes, you are correct. We simply cannot know with certainty. But a preponderance of evidence does seem to indicate that Zimmerman suffered a sustained assault that night, and it is a cold hard fact that Martin was shot dead, the bullet killing him only moments after impact. By 'sustained' I simply mean it was more than a single punch, but a series of punches or various other strikes.
It is not unreasonable to infer that Zimmerman shot Martin in self defense. If Zimmerman initiated the coercion, then Martin attacking Zimmerman was indeed self defense. But based on what we can know with certainty, it would seem more reasonable to believe Zimmerman's actions were more of the self defense kind, than Martin's.
Bottom line for me is that the Jury made what I believe to be the best choice under the circumstances. The state must prove its case.
As a strictly Libertarian question, it seems unreasonable to demand Zimmerman be punished. For as you say, we can not know.