Quote - Allow me to point out the obvious here, but Acadia didn't see what started this incident.

Given that Acadia didn't, we don't know all the facts. For all we know the store security may very well have been protecting everyone from a crazed woman who'd brandished some sort of weapon in the store.

Wich would mean, in such a situation that the force was not "excessive".

 

Not exactly. The head is considered a "no strike" zone for police and security. (If this guard had any kind of force training, he would know that. If not, he should not be working in an undercover capacity.) There's also the use of force, which basically should be used to stop the action. If the woman had brandished a weapon, the force would be focused on the weapon (stopping the action). Any lawyer would use Acadia's testimony to good effect, as the guard's actions were excessive.