Bara to Koroshiya

The manga centers on Ari, a girl who has just had her parents killed in front of her, with the hired murderers intent on killing her too. Fleeing from the murder, tired and desperate, a figure arrives to save her, a professional hitman named Kei Bishin. Together with Kei, Ari resolves to get back at the organization that killed her parents.

(Source: Anime News Network)

Note: The series was originally announced under the title "Karasu to Usagi" (カラスとうさぎ).