What is evil

In an Okami/ Samurai Jack Crossover, Aku and Yami/Akuro are not to be depicted as seperate entities but rather previous incarnations of the same being. Much like Yami (Tokoyami no Sumeragi) and Akuro's (Akurō-Ō) names in the english versions of Okami and Okamiden, Aku's name would be an abbreviation for the full name: Toori Akuma.

By combining the lore of both shows, This would create a sort of alternate universe that differs from the timelines of Both Samurai Jack and Okami.

This would also be a way of showing that no matter how many times you try to destroy evil, Evil will always be there and that Good and evil cannot exist without one another.