Yuusha-goroshi no Moto Ansatsu-sha.: Mushoku no Ossan kara Hajimaru Second Life

Abandoned by his parents at a young age, Juu has assassinated people for over 30 years as part of an assassin organization. In return for a break from the assassination business, he is given the final mission of assassinating a hero. This is the start of a second life where a worn-out middle-aged man with no redeeming qualities other than assassination can finally grasp "freedom"!

(Source: Champion Cross, translated)