The Boondock Saints is a film about two Irish brothers living in Boston who become crime fighting vigilantes who hunt down and kill criminals - with religious vengeance. It is about as violent as you get in terms of action films. So if you love super violent action movies then you will definitely enjoy The Boondock Saints.
The sequel, All Saints Day, is also excellent.
Both are totally worth watching. An easy 4.5 stars out of 5.