Zimmerman has already been proved guilty, now it's just a question of punishment.

[Azathoth]

81. Are you for real?

I just googled the terms and came up with tons of freakin' hits.

Here's the crude Cliff notes version: justified acts do not harm society (in fact, they are endorsed by society) and thus are not criminal. Excused acts are essentially criminal acts for which society decides that individual defendants should not be punished or otherwise held accountable. Practically, both are affirmative defenses that can lead to the same result: "not guilty."

Self-defense is a justification. In fact, it's *the* prototypical justification. Successfully asserting self-defense means objectively that no crime was ever committed. Insanity and most types of duress are the classical examples of excuses. "Yes I shot that dude in the back, Your Honor, but I was schizophrenic at the time!" The exact dividing line between the two is slightly murky and the subject of a lot of scholarly wrangling, but the distinction is basically common-sense and well-established in legal theory.

Your OP is therefore wrong both in theory and in practice. Practically, Zimmerman is not guilty of anything yet. He also hasn't admitted anything for which he could be theoretically, philosophically, or rhetorically "guilty" because he's asserting a justification and not an excuse.

...he can't merely state that he did so then sit back and force the prosecution to prove otherwise.

That is exactly what he can do. The jury is entitled to consider any evidence that he presents of self-defense, including his own testimony. Some defendants get off simply by taking the stand and testifying that they defended themselves. I'm not versed in the nuances of Florida law, so here's a relevant Google result:

http://www.ohioverticals.com/blogs/akron_law_cafe/2012/04/zimmermans-low-burden-of-proof-on-the-issue-of-self-defense/