I don't know what happened, so I'm not going to comment on guilt. But witness testimony that Martin was straddling Zimmerman and punching him isn't indicative of anything except that at that point in the fight, Zimmerman was losing.
This does not establish or remove self-defense as justification. It could be that Martin attacked him, or it could be exactly as Dubyamn described, or some third thing.
The crying for help thing also doesn't really establish anything. If Martin was the attacker/instigator, he could still be calling for help when he saw the gun. If Zimmerman was the attacker/instigator, he could still have been calling for help because he was getting his ass-kicked in a fight he started.