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 !

An obsessed vigilante has to face the errors of his past when the son of one of his victims infiltrates his gang and seeks revenge on his family

I guess the higher stakes is he's gonna be killed by the commando unless he admits his mistake and shows the commando remorse...

he's gonna either be killed by the commando kid who's come for revenge, or be arrested and go to jail for the rest of his (old) life

What happens if he fails to face the errors of his past? What is at stake?

It needs a coma in there, as well it would be good if we had an extra word to explain the character, such as - An obsessed, (guilt ridden or single minded) vigilante confronts his sinfull past- blah blah
And as well does this mean there was a gang of vigilante's or was he just part of a gang?