After he kills an innocent man, an over-zealous activist, who’s become a hitman for environmental causes, decides to defy his controller, his best friend, and to make amends with the victim’s daughter.

This is what I can see, but hitmen are KILLERS – rarely portrayed as people who care if their target’s innocent or guilty.
“When killing an innocent oilman, an activist hitman seeks revenge against his controller in order to make amends with the victim’s daughter”

Take care.

Why will she not just call the police and have him arrested for murder? That's precisely what I'd do!