Father Kills Bad People, Makes Son Help

I remember watching a horror movie a long time ago, but I can't remember the name!

The film is about a boy who lived just with his dad (I think his mom was killed by a drunk driver or a robber) and the dad had this little shed behind the house where he would kill people who did bad things like; rape, murder, theft, etc... The dad tried to get the son to help out but he wouldn't.

Eventually the kid killed the dad because the dad was doing wrong, kind of fulfilling his fathers wish (I think).

That's all I really remember... Does anyone know what I'm talking about?