Rental Murderers: Agents of Revenge

Mashiro has been the target of bullying ever since he started high school. He tries his best to deal with the physical and emotional torment, but his days turn even darker when his younger sister, the only family in his life, goes missing. Discovering that the bullies are involved in her disappearance, Mashiro finds a personal revenge service called "Rental Murderers" and seeks their help to get even. However, he is unable to contact them after he pays their fee using all the money he has saved up for his sister's college fund and assumes he has been scammed. Pushed to his breaking point, Mashiro attempts to end his life but is luckily saved by an unknown woman named Kiyoka who suddenly appears. She claims to be an agent sent by Rental Murderers, and...thus begins an epic journey of revenge!

(Source: MangaPlaza)