An obsessed vigilante has to face the errors of his past when the son of one of his victims infiltrates his gang…

Like it dude!

Currently he does use his daughter to get in the gang, but then falls for her, which makes it hard for him to enact his revenge on the Vigilante.

In the end he helps the Vigilante find his son's killer and achieve his goal.

with revenge it doesn't have to be straight out and kill, an eye for an eye.He could torment him emotionally, dating his daughter and making her hate her father etc. That would be worse than death...

That's great. Would change the plot drastically with 'seeks revenge on his family' but sounds like a good film !