1) He would call police to establish his position that it was self-defense.
2) I don't think people are suggesting that he went out looking to kill a black guy, but rather that he made an assumption that Trayvon was a criminal because he was black, and was determined not to let this one "get away".
3) Perhaps he didn't start out with the idea of killing him, but just to hold him for police, but ended up shooting when TM didn't cooperate.
I don't know IF he set out to kill someone just because they were black, but to me from the things he stated and his tone of voice on that 911 call to dispatch I'm inclined to believe he was on a mission to harm...JMHO