Densetsu no Ansatsusha, Tensei shitara Ouke no Aisare Suemusume ni Natte Shimaimashite.

"Babuba, Babubabubabubabu!?" (Am I going to be a baby!?) Thoth, a legendary assassin in the underworld, was sold by his parents at a young age and was raised strictly as a killer, so he never knew the love of a family he lacked. Suddenly, he saves a cat and dies, but after his death, he's destined for hell for taking countless lives. However, because of one good deed, saving a cat, he's allowed to be reincarnated and is born again as a royal princess, Patricia. Then, God commands him to save the royal family, his family. He was supposed to be reborn as a helpless baby, but his assassin skills haven't faded, and he continues to battle killers one by one. Gradually, Patricia develops a desire to protect the family that loves her simply because they are family... (Source: Shueisha, translated)