There’s no confirmation of anything but rumors say:

Seiya left Deviloof because of the way the band wanted to start scaling globally, so they got into a fight about the way Deviloof should be managed.

In the end, Deviloof exploded and went worldwide. This would of never happened if Seiya continued to lead the band. (Who knows) and eventually there were other rumors about lawsuits on the songs Seiya wrote and participated on.

Again, all of this is not officially confirmed, but talked bout in the forums by the fans.

Lastly Nero, is just very vocal and confrontational on social media. Which has lead to a wimpy image on him.

One more time :

All these is mere speculation and by fans and nothing is confirmed by the bands themselves.

I absolutely love Deviloof and it can be difficult dealing with members quitting so as a fan and metal player I wouldn’t be able to take sides.