Read it in one go. I couldn't sleep anyway for two reasons: first, it's just TOO AMAZING, the way of killings are on point, the art style is beautiful, there's enough mystery to keep you reading, the plot twists are unpredictable... second, it ENDED.
Btw this is the prequel of the first manga, "Ousama Game". Read that one first bc this is about the origin of the "Ousama".
Ou-sama Game - Kigen