I feel certain that AK was said by Voldemort. He always went for the forbidden curses (and meant them). The only curse I remember Harry using was “Crucio” and exclaiming that its true they only work if you really mean it. (Can’t remember which fight it was though.)
Remember, Harry kept using “E” and kept getting told to stop using it because everybody else was trying to kill him. Harry really never wanted to kill anybody, even Voldemort in the end.... remember the “icky baby thing”? I think he was starting to feel sorry for Voldemort because he knew (finally) that he had all the power and protection thanks to love. Voldemort didn’t have a chance and Harry knew it.