"You can not kill that which does not exist" (Buddhist saying). Mu means emptiness in Japanese and it describes a state of being in which the person who has obtained it projects neither sakki nor fear. At this time they become imposible to strike down as one must return your sakki before you can attack. Only another who has also attained Mu can slay them.
The path to obtaining Mu is described in Dark Horse's "Lone wolf and cub" as follows,
"Met the Buddha, kill the Buddha. Meet your parents, kill your parents" (every 14 year old's dream I'm sure). Essentially one who has obtained Mu has conquered their fear of dying.