Vote for the villain origin you like more.
Round 1, first battle:
Originating long ago in the depths of space as a formless evil Aku was banished by the gods Odin, Ra, and Vishnu in order to protect the universe from his influence - yet a small shard of the great shadow managed to survive and flew off into space - for many years this shard continued flying through space until it eventually landed on Earth, causing the extinction of the dinosaurs in the process.
Many years later the site on which Aku landed would become the ancient island of Japan, which Aku infected like a horrible disease - creating vast forests of evil in the land and threatening to consume it, eventually Aku's threat became so great that Jack's father decided to rid the land of the demon once and for all using magical-oil made by Buddhist monks, however when he fired an arrow coated with this oil into the evil entity it didn't destroy it but rather gave it form and sentience, thus forming Aku as a humanoid threat of unstoppable power.
He used to be an undercover SHIELD agent who injected himself with the serum giving him the photographic reflexes. After seeing somebody performing an action, he can perfectly copy it (for example, copying the fighting styles of the various heroes and villains by seeing them in action). As a side effect of the serum, he developed an amnesia and created his alter-ego 'Taskmaster', the mercenary working for the various criminal organizations.