I doubt this officer had just arrived on scene and decided to attack the guy on the floor for 'no reason'. He's probably not 'evil' or an inherently bad person.
More likely he'd been involved in the incident, had seen this thug attacking and injuring several colleagues and lost control for a few seconds whilst still full of adrenaline.
Either way. He's still unfit for duty and needs to be fired and charged. If you're unable to keep a handle on your own emotions/anger/actions in extreme circumstances it's not the job for you.