Just a heads up, none of the guards die. That’s confirmed in the second-to-last panel in Page 38, where we can see the guards most certainly alive, albeit a bit drained. @Kisshouin
Did they just spoil the entire story in this 2nd chapter?
He's actually the new demon king AND the old one is really dead. It's not a curse but the demon king's power? Later he becomes more powerful and gets holy resistance? So he gets all the girls?