Huber and Rossenbaum were both armed when they were shot.
They both grabbed Kyles rifle. The medical report details how Rossenbaum has burn marks on his hand from grabbing the muzzle.
You put your hands on someones rifle, you are now armed. Kyle testified that, among his many concerns, was if he was rendered unconscious, they would take his gun from him and threaten other people.
Anthony Huber was already in the process of stripping Kyle of his gun, the sling coming off his shoulder, when Kyle shot him.