But, Trayvon isn't here to give his account.<quoted text>
You asked why Zimmerman would engage if he was in such bad shape. That implies a choice to engage. There is no evidence that he made such a choice. From his account, it appears that the engagement was thrust upon him.
Again, what we know is that Zimmerman was chasing and Trayvon was running away.
And Zimmerman articulated some determination to keep this one from getting away.