I Became a Demon and Killed Every Cultivator

[Decisive and ruthless protagonist + some angst early on + bloody revenge + monstrous protagonist + ultimate wish-fulfillment]

“Master, why are you so uneasy?”

The master shook his head. “Man proposes, but Heaven disposes. The more you fear something, the more likely it is to happen.”

“Your plan has failed. He's coming to kill us.”

“Impossible. No one can defeat us! Our cultivation is…”

“He may no longer be human. He may be a god.”

“A Demon God.”