A grieving London gangster recruits the tough new boyfriend of his suicidal daughter to help him hunt the killer of his son. But when he discovers her boyfriend is the son of one of his victims come for revenge, he must risk his daughter's life to take him down.

or:

An embittered vigilante who wants to execute his son?s killer recruits his daughter?s new boyfriend - a security expert - to help crack the alarm of his high profile target. But when the new recruit turns out to be the son of a past victim come for revenge the Vigilante is forced to make a choice: stop his bloody rampage of vengeance or save his own family from being destroyed.

An embittered Vigilante?s been out killing child-killers ever since his son?s murder 5 years ago; but when, on new information, he goes after the real killer and puts his wife and daughter in danger he?s faced with a choice: continue with his bloody rampage of vengeance or save the lives of those he loves.

Here's another version with the main antagonist in the logline. The above logline has two other opponents in the logline (opponents in this story are threefold as per Truby).

"When an embittered vigilante recruits a tough teenager for his next hit and discovers he's the son of a victim come for revenge he must make a choice: abandon his moral code and kill an innocent or pay with his life."