Angel -sacrifice is also a part of love

The story takes place in the year 2040 were a Assassin's group called snakes is famous for the assassination. A girl comes to them and gives a contract to kill her but the drift comes when one of the Ace' s. Who is going to be next leader of Snakes falls for this girl right after seeing her. Which leads the main character to abide by some rules inorder to save the girl. Will The ace save the girl or not, and who are assassins forms the rest of the story.