I have always wondered why hitmen in movies always want to right their wrongs or give it up and make amends. You would think if they had that kind of moral code they wouldn’t have become hitmen in the first place.
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As for the logline: “…he will have to do what he swore he would never do again.” is so vague it tells us nothing about the story.

Why is he retiring? What event caused him to retire. “When a hitman accidentally kills a child…” “When a hitman is hired to kill the love of his life…” give us something to work with.

Let us knoiw what he will never do again… “He swears he will never kill again…”

Then give us a reason why he is breaking his vow “…But when a vicious gang kidnaps his beloved niece he must do what he swore he would never do again.”

You have to give us the event that make him change his ways, tell us what he swears he will never do again, then tell us what even causes him to go back on his vow. (All within about 30 words:)

Hope that helps, good luck with this!