The Regretful Novel's Male Supporting Character? But I'm a Great Villain in a Fantasy World

[Urban Fantasy] + [Decisive and Ruthless] + [Cold-Blooded] + [No Female Lead] + [No Clichés] + [No Beating Around the Bush] + [Villain] + [Mastermind] Gu Changge, the great Villain of a fantasy world, transmigrates into a regret-themed novel and launches a massacre of the Sons of Destiny! What? The heroine regrets getting together with him and wants compensation to give to her ex? Kill! What? The second female lead wants to work with him to win back the male lead's heart? Kill! Gu Changge completely ignores the usual formula. He won't waste time getting entangled with the original characters. He just kills them! On top of that, he gets the Not-Quite-Human System, which lets him issue missions to the protagonist! All the rewards for completing those missions go straight into Gu Changge's pocket. In other words, whatever Gu Changge wants, he just needs to give the protagonist a mission for it. “Ding! The Infinite Demon Art has been issued.” “Ding! The Infinite Demon Body has been issued.” “Ding! The Infinite Demon Sword has been issued.” Protagonist: “System, where did my rewards go?”