At the end of Samurai Jack S3E4 after fighting with Jack, Aku is in such a weakened state that he is no longer able to fly, nor even run efficiently, by the looks of it, he was reduced to just a... frog? Mouse? Barely capable of jumping away.. and Jack just let him get away, why? The scene in question is around 19:00 here: Related scene
(I don't see why Jack couldn't have
killed Aku 2 seasons early
in this very episode.)