I have a theory that the Nixels are actually the GOOD guys while the Mixels are the bad ones.
First of all, let's check out the Mixed Up Special. You remember the Nixed Mixels, right? MAYBE THOSE ARE THE TRUE FORMS OF THE MIXELS! They act a lot like Nixels! They might have been transformed into colorful slaves by the true evil overlord known as FLAIN. Maybe the Mega Nixel Mixel Nixer was an antidote for the slaves! Also, the Mixel Mountain looks all dangerous, and it has a big Cubit on top. In my view, Major Nixel put the Cubit on Mixel Mountain so Flain & his slaves wouldn't get the Cubit and make everything fun & creative.
Another example is Nixels. Flain & Seismo were having a lava slide until the Nixels came. They were probably getting REVENGE on Flain for making their Nixels into Mixels. When the Flain & Seismo Mix appears, the Nixels are seen SCREAMING! Not just a scream, but a GIRL scream! Flain was terrifying & torturing the Nixels so they could turn into a lava slide.
The final example is Another Nixel. When the Nixel with the backpack came back, he came with a Mix! He might have reminded him about the past so he betrayed the Mixels and became like Nixel!
Also, look at this picture. THE MIXELS WERE MURDERING THE NIXELS IN MIXELS MANIA!!!!!!!!!!!!!!!!!