@VariantJ I think you are miss crediting both Zil and the world building so far. The author already established the world well enough to gage how most demons would respond to this information "More deaths than expected? Man this patch is truly such a weak little babies" and like this is the best case, in the worst one, the other powerful families would throw the blame of the casualties on the Rage family being weak and away from the battlefield for a long time or Zil inexperience as a commander, which would only increase the infighting among demons.
I also think you guys are underestimating the threat of the demons a little bit here, I know we spent a relatively long time with them that for use they just seems like a bunch of goofballs, but demons in general are such a broken race in combat that they literally needed all the other races put together to make a powerful enough external enemy, so they didn't imploded from within, but like even after that, the Human Alliance still are being destroyed on daily basis. Remember that the war should have ended in a few short years if they were willing to go all out. So characters that could make the demon race, advance culturally are the biggest threat to Zil's plan of putting the Demon Race in a new dark age, the guy that Zil murder a few chapters ago was possible a Demon King level threat in the making.