"The Boondock Saints" is an extremely over-hyped vigilante thriller that contains no depth beyond its initial macho revenge fantasy, but, despite moments of painful camp, doesn't have the sense to go all the way as a comedy. It would better serve as a satire on America's obsession with Machismo posturing and the view that violence is the best way to solve problems than the self-important bloodbath it becomes.
I'm not adverse to revenge movies, even extreme ones. "Taxi Driver" featured Travis Bickle blowing away pimps and thugs, but it was more of a character study than a vigilante movie. "God Bless America" trivialized violence, but it was a satire, and a good one at that. "Dead Man's Shoes" was a powerful statement on the consequences of violence.
I don't have any problem with violence in the media at all, except when it is portrayed as an easy way to solve real-life problems. People, I cannot stress this hard enough -- there are consequences to violent retaliation and vigilante justice.