Thats a young teen voice calling for help.
READ THIS SLOWLY
Let's say it was Trayvon yelling for help why didn't he yell he has a gun he's going to shoot me. Why just yell for help?
Perhaps at that point he didn't know that Zimmerman was armed. He just wanted someone to call the police because this dude followed and then chased after him. The police report said that he had a gun and holster, which means it was probably clipped onto his belt.
That's still speculation, but your strawman seems to have holes.
All we have are the known facts. Zimmerman chased Martin, even though the police told him not to. Zimmerman has a documented history of violence, and was even forced into anger management classes by the court. Martin was on the phone with his girlfriend at this time, and she told him to run. It's unlikely that a 150 lb kid would go out of his way to attack a 200 pound guy, armed with Skittles and iced tea, unless he feared for his life or safety.