Why is Sieg freaking out over the King trying to execute the Demon King as if that's something he could actually do? Guy has no power or ability to do this so taking it seriously is stupid. Sieg throws a man baby tantrum then says I'll make a beast kingdom, which already exists. He has spent the whole of the series so far there. I assume this is a mistranslation and it should be something more clearly different from Beast Kingdom, even Beast Union doesn't really fit because that's functionally what the Beast Kingdom was anyway. Making Sieg the Demon King because he said I'll become the Demon King is stupid as well.
This whole plot is stupid and sets up things only to immediately ignore it anyway. The Beast Kingdom has a magic barrier around it that serves no purpose since everyone just wants through it whenever they want so what is the point. Elf hero hunts an ancient dragon for no reason as if this would not cause issues with the nearby Dragon Kingdom. No reason for the Demon Princess to forget to put the barrier back up around the Demon Castle. Even if she did, the evil traitor guy almost certainly would've put it back up to trap her back inside. Goddess was a moron to ever choose the elf hero or the human hero as heroes given their personalities.
The art on this series is so bad and inconsistent. Tiny heads, thick necks. Sieg in chapter 1 and Sieg now barely resemble each other. Body type is totally different.