I love films and comics about hard-nosed vigilantes - the clear delineation between good and evil and a simple way to solve the issue always appealed to me. I abhor any type violence in the real world, as it is never constructive and is in opposition to what Christ teaches us about loving our enemies, but in the fantasy world of books, movies, and comics, it is an ingrained part of the heroes' world and a legitimate method of solving problems. Indeed, it is often the only way in a violent game world to stop something from becoming way, way worse. The real world doesn't typically let crime slide so out of control that we have the complete insanity that you would see in something like Hobo With A Shotgun or even Death Wish. And games are an escape from the real world, where we can see things in black and white and the choice to stop a criminal by any means necessary is simple.
That being said, I've come up with the germ of a campaign idea for D20 Modern or maybe Savage Worlds (if I feel like learning a new system this would be my first choice, Alternity coming in close behind it). It would take place in a semi-post-apocalyptic (how's that for vagueness?) city at first, perhaps like the cities of The Warriors or Escape from New York - in other words, it's the points of light idea from 4E grafted onto a modern/near-future setting. The PCs are the vigilantes or bad guys gone good, who are sick and tired of the chaos of endless gang warfare and being hassled on the street. Bodies are piling up, and the only way to solve it is to strike. Crime lords and a feudal gang system have to be dealt with, as well as the equally sick government that allows things of that nature to thrive and become partners instead of enemies. Serial killers operate in the open, and other varieties of human monsters are on display as well.
For material, I'm going to mine movies like Death Wish, The Warriors, 1984, A Clockwork Orange, all 3 Punisher films (including the one with Dolph Lundgren), Escape from New York/L.A., Cyborg, Terminator, Mad Max, Day of the Dead, and various less-well-known vigilante and near-future post-apocalypse films. Judge Dredd comics and the corresponding D20 game by Mongoose are going to get mined for anything useful as well. I may have to inject a little extra testosterone into my players' bloodstreams. One or two of them may lack sufficient quantities for this to truly get them excited.
The cool thing about all of this: when I run out of mundane-world ideas or the PCs solve the world's ills, I simply turn to the back of D20 Modern and choose a campaign outline and gradually introduce out-of-control weirdness onto the streets in the form of D&D monsters. Ah yes, the chaos never ends.