Boku ni Korosarero

Yuu Yakushimaru is a middle schooler who has been skipping school and spending his days harboring hatred toward his bullying classmates, fantasizing about slaughtering them all someday. When he gains a the power to kill them and discovers his targets are actually monsters he can kill without remorse, his relentless killing spree becomes unstoppable! And so begins a story of non-stop horror and suspense!

(Source: Kodansha, translated)