Aside from not using the gun in the first place, this actually fits the scenario? Rose beat Akashi, thus Rose is now fiance because that's the deal. Presumably, until a wedding, title is still up for grabs. So he kills rose, title is now his.
Wait if he had the gun the entire time... Couldnt he have used it on the MC considering how apparently oni power dont make you bullet proof? This is just getting incredibly stupid by each chapter and not good stupid just stupid stupid.
Hoping for Kiko to do something useful or cool next chapter after seeing this idiot asshole take out two of her friends right in front of her, but so far this manga has played it safe and stupid.
Even if Akashi was confirmed dead... if you're the king of the demon world, why wouldn't you have any use for an extremely powerful assassin anymore? Not like people would instantly obey you, you'd have people gunning for your position that need to get put down, just like he's doing now.