Almost all state laws (I'd be surprised if it WASN'T all) either directly or indirectly allow for use of force to protect another, including deadly force when necessary. This situation seems cut and dry, as described. It's clearly an unjustified assault that will obviously result in grievous harm or death. I know of no place in the US that intervening with extreme force would be prosecuted. Maybe some jurisdiction somewhere, but not on any large scale.
I sure hope your right.