Shibuya's wish was clear. Not more death games in the galaxy. And can't remember exactly If Mothercon did or not took their innocenses as part of the rituals, that should be something to remember forever rather than the betrayer who isn't gonna help them get revenge. And with how the people is...
Shouldn't the ex-slaves want to kill the little brother instead? Akiba barely did a thing to them but little brother probly had fun time or something with the "ritual" he did to them.