The "large armed adult" was 5' 9". The "unarmed teenager" was a 6' tall football player. As for being armed, that didn't help until Zimmerman pulled the trigger. Assuming that Trayvon attacked Zimmerman and posed an immediate danger of inflicting serious bodily harm, pulling the trigger was both legal and exactly the right thing to do.
Police report: “While I was in such close contact with Zimmerman, I could observe that his back appeared to be wet and was covered in grass, as if he had been laying on his back on the ground. Zimmerman was also bleeding from the nose and back of his head.” It's going to be unimaginably difficult to prove anything other than self-defense.
“Proof” does not figgure in this case. The is a case of legal politically inspired lynching.The strong message that is to be sent to Whites is that if attacked by a Black and or a group of Blacks, should you successfully defend yourself, the law will come after you.