Had Martin kept walking home, it wouldn't have happened either. (He could have been there in between 30 and 60 seconds.)

Nor if he hadn't gone back, broken Zimmerman's nose, and "pounded" his head on the sidewalk.

Zimmerman didn't chase him, at least according to the phone-call timeline, and didn't strike him, at least according to the autopsy.

Not every tragedy is a crime.