The Big Bad is...Either Villagillo or Moid
The Power of Friendship will turn out to be a VERY VERY bad thingAfter all, the Vox units are practically fueled on the stuff, and that certainly amplifies between the three pilots. The WHATEVER THE HELL Vox Aura did was certainly within anybody's evil intentions. It's likely Villagilio did his best to break Muginami with full intent to make the other two Vox units do the same thing.
The Vox were...Originally made to terraform planets but the pilots lost control and ended up destroying most of civilization on Earth at the time. The true nature of the units were lost to history out of fear towards their power until they are used as they're supposed to at the end of the series.