It has yet to be established that there was a murder. Zimmerman has been charged but not convicted. If he was acting in self-defense, he did not commit murder. Trayvon, on the other hand, appears to have been trying to kill Zimmerman. If he had succeeded, that would have been murder.
Oh, I completely agree with you! I just got caught up in terming things the way the prosecution wants it to be known. I have no doubt that Zimmerman acted in self-defense. That bloody nose and face photo told a lot more of the story than the media was willing to let out.