User blog comment:MemoryAngel/Are the Oni the true source of all darkness and evil in Ninjago?/@comment-35373528-20180225011122

I think so.

I think when the first spinjitzu master created something, he put oni power in it by accident, causing evil in the world.

And this goes further when his sons were born and Wu got creation (Good/Dragon) and Garmadon got destruction (Evil/Oni)

So that there would be "perfect balance."