The mask is a spit shield. It is used for suspects who may spit blood or bite. This guy was not even under arrest when he grabbed the cop's gun and shot him. He then shot out a window and jumped 60 feet to escape. The cops stated that it took them several minutes to subdue him, once they caught him. He may well have taken a beating when they were arresting him.
The sub human wasn't under arrest but was in for questioning for alledgedly stabbing an 84 year old woman...quite a class act.