Diary of a Hitman tells the story of Hitman Dekker, a professional killer who has been hired to eliminate Jain, the wife of his client, a wealthy businessman. Dekker is a seasoned killer who has successfully performed many hits, but as he begins to watch Jain, he becomes increasingly conflicted. He finds himself drawn to Jain's gentle nature, and her love for her mentally disabled sister.
As Dekker lingers in Jain's vicinity, he begins to question whether he can actually go through with the killing. His inner turmoil is further complicated by his relationship with his handler, who constantly pressures Dekker to finish the job quickly. Meanwhile, Jain's husband is growing increasingly impatient, afraid that his affair with a younger woman will be exposed if Jain remains alive.
As the deadline for Dekker's hit approaches, he must battle his moral compass and decide whether to follow through with his assignment or protect Jain and her sister, even if it means going against his client's wishes. In a final, climactic showdown, Dekker must face his own morality and past mistakes before making a decision that could affect the lives of everyone involved.