The Boondock Saints (1999)

Brothers Conner and Murphy MacManus take to performing their divine duty against the Russian mob. They massacre a bunch of unsuspecting Mafioso in a scene of absurd violence, then they let more blood in a mass killing of porn-shop customers. They then see it as a calling by God and start knocking off mafia gang members one by one. Willem Dafoe plays the detective trying to figure out the killings, but the closer he comes to catching the Irish brothers, the more he thinks the brothers are doing the right thing.