Trayvon started the physial altercation. Prior to the physical altercation there was mere verbal altercation. Even if Zimmerman started the physical altercation (which he didnt) if the other person causes what was originally a very minor squabble to become potentially deadly, the other man is entitled to use deadly force. That is common law. Alternatively, if Trayvon started the physical altercation, if Zimmerman reasonably belived he was in danger of imminent deadly or very serious bodily harm, he was within his rights to use deadly force.
If a, he used disproportionate force, or b, it wasnt objectively reasonable to believe that his life was at stake then it results in voluntary manslaughter; ie, IMPERFECT SELF DEFENSE.
No matter what, there is no possible way you can get to 2nd degree murder.