Thousands of years ago the world was a very dangerous place. No one knew who they could trust, so they trusted no one. A stranger approaching your home was viewed with great suspicion and rightfully so. It was better to pull out your weapons, kill first and ask questions later. It was how you and your family stayed alive.
Of course, this created great difficulty for strangers who bore no ill will. Innocent travelers constantly had to prove their were deserving of their own lives. So, in the interest of civility, a system had to be worked out to make it possible to welcome good strangers while warding off the ill intended.
It became common knowledge that there were certain behaviors that would allow you passage. Deviating from these norms would invite the pulling out of swords and spears. When you passed through someone else's land, you did so with great respect for them. You walked as if the land held no interest to you because you were only passing through. You removed any masks, helmets or hoods to show you were not trying to hide your identity, that you were a person of good will. If you encountered the inhabitants of the land, you understood that all decisions were theirs to make. They would decide if you seemed safe to them. It was your obligation to identify yourself to them, state your intentions and ask permission to pass through their land. If they granted permission, you could pass unmolested. If they refused, you respectfully backed away and looked for a different path to your destination. At all times, you showed deference to them, it was their land, they were in the right. Failure to follow the correct behavior could get you killed. Following the proper procedures could make it possible for you to make new friends or at least come to an understanding with the people who had every right to protect what was theirs.
Trayvon Martin was taught none of this. His parents, teachers, clergy all failed to teach him to have respect for others, to show deference where deference was due. He walked through a neighborhood that was not his and acted as if he had every right to do so. He stopped and looked at houses as if the houses held some interest to him. He wore a hood, a centuries old sign of criminal intent to hide one's identity. When confronted by a man who lived in the area, he refused to answer questions and demanded to know why he was being followed. When the confrontation turned physical, Martin seemed to be winning at first. He had Zimmerman on the ground and was pummeling the older, shorter, less athletic man. Zimmerman, defending his neighborhood and now in fear for his life, could feel blood escaping from a head wound. At some point in the fight, which he now believed to be a life and death struggle, he realized he was outmatched and likely to suffer further harm, possibly to the point of dying. He pulled his gun and defended his home, his neighborhood, his life against an attacker.
Trayvon Martin could have avoided all of that by simply saying, "hello, my name is Trayvon. I live a couple of blocks away and am just coming back home from a store. Would you like some Skittles?" Martin's defiant, in your face, "why are you following me" attitude was exactly the wrong way to deal with a man who was defending what was his. I am not saying he deserved to die. I am saying his lack of civility, his lack of understanding of how the world works caused him to make very bad decisions, which is what lead to Zimmerman's decision. A tiny amount of civility on Martin's part would have kept him alive.