There are many uses of excessive force out there, but if the basic story holds up I don't think this is one of those.
Zimmerman never cornered the guy anywhere. Looking into private residences with a bag of tools (a break-in bag) and someone looking into a store window that purposely displays items in those windows, are not the same thing.
A security guard (who is also a cop) is not a guy who decided NOT to be a cop but was just a ccw neighborhood watch volunteer, not even on volunteer time, running an errand at the time, is not the same thing.
This guard reverted into COP mode. Security guards are not cops and the jurisdiction is different and hunting perps down off the mall in an alley is I believe, over the top. I believe it showed he was in COP mode and not security guard mode andymore.