Ari to Hachi

Tamura is a forty-six-year-old middle school graduate who works spot-gigs at his own pace. When he returns home, a beautiful wife, a handsome son, and a fluffy doggo are waiting for him. Though his life isn't flashy, Tamura lives a deeply fulfilling daily life—yet he possessed another face: that of a super-skilled assassin. Seizing the opportunity of his son reaching adulthood, Tamura resolves to wash his hands of the assassin business, but the organization's boss hands him a certain harsh condition—The ultimate assassin papa's final major job.

(Source: Kodansha, translated)