I Chose to Become a Villain After Regressing

I returned back in time. In the future, the world will be overrun by zombies and monsters. All those years, I held myself back, only killed when I had no choice left, befriended people who backstabbed me and fell in love with a monster. No more. If I can't be Mr. Nice Guy in the apocalypse then I'll be everyone's worst enemy. I'll be the villain that everyone fears. Warning: Sex (although not explicitly shown), Murder, Torture, etc. THE MAIN CHARACTER IN THIS NOVEL IS A REAL VILLAIN. WARNING!!!