Shark in a Bottle.

Guy Normal (probably not his real name) is clumsy, has amnesia, and has fallen into the hands of an unscrupulous man who arranges killings for hire. The Arranger trains Guy to be a killer (and there's evidence that Guy perpetrated a mass murder in an Arizona post office), then sends him to do various jobs. Although Guy bumbles, the jobs get done. But Guy rebels against this life, tries to reconstruct his past, and falls in love with the Arranger's mistress (who also has no place else to go). When Guy becomes expendable, his days seem numbered. Can Guy outsmart the mastermind and the highly skilled assassins sent to kill him?