You keep, conveniently, leaving out the fact that this black man (which is not a race BTW, which makes your whole argument invalid. A Black man from the Congo is not the same race as a black man from the Dominican Republic ) was in a gated community to which he did not belong. Not only did he have a physical similarity to those that committed previous crimes in that exact place (not saying that makes him guilty), he was, in essence, trespassing. Is a gated community not gated for a reason? As a community watch member of that GATED community, Z would have been perfectly within his rights to question T. In fact, that's what that position is supposed to do.
He didn't live there normally but he wasn't tresspassing, he was a guest of someone living there. The fact that Zimmerman is unaware of who the guests are there kind of makes him seem less competent as a neighborhood watch guy. I mean how would you like it if you lived in a similar place and a young friend or relative got killed simply because someone thought they looked suspicious and didn't know that person was your guest?