Armitage plays Sir Guy of Gisborne, second in command to the villainous Sheriff of Nottingham.
Sir Guy stabbed Marian, played by Lucy Griffiths, in the show's Season Two finale, which aired Saturday night on BBC America.
Long in love with Marian, Sir Guy killed her after she declared her heart belonged to the outlaw Robin Hood, played by Jonas Armstrong.
"It gave me sleepless nights," Armitage said in an "Inside Look" segment that aired during the finale's commercial break. "I just didn't know how I was going to play that scene. I didn't want him to want to kill her. I wanted him to kill the thing that was coming out of her mouth -- the thing that she was at that moment. All of his hope for his future was absolutely exploding into nothing. I think that just completely destroys somebody in a moment."