A grieving, self-righteous Vigilante must confront a fatal error before burying his hatchet in the killer of his son…

A self-righteous gangster-vigilante, no closer 5 years later to tracking down his son's killer, takes on a new recruit who has vital information - but only at a price - he learns that he was responsible for his son's death...

Thanks. However, the protag's a gangster who turned vigilante when his son was killed so he's been hunting his son's killer for the last 5 years. It's the new recruit, the son of one of his victims, who gives him new info which finally leads him to his son's killer. But the new recruit only decides to give him the info when the gangster-vigilante shows remorse for killing his dad...

Per Geno, you're still holding back on the hook, which is the father being responsible for his son's death. And it doesn't add anything to your logline to pile on the adjectives.

To me, "bury the hatchet" implies a horror film. Is this a horror story?

My attempt:

When a grieving father learns that he was unwittingly responsible for the death of his kidnapped son, he takes matters into his own hands and tracks down his son's killer.

Best of luck!