Someone must have done this.
For a one night adventure, it might (emphasis on might) be interesting to run actual murder hobos, possibly as the PCs, possibly as the villains (depending on where you stand).
Say, set it during the Great Depression, about 1933. The PCs are a set of hobos who are awakened to their greater purpose, which is to whack the evil dudes, and they know who the evil dudes are...the wealthy, of course. The corporate fat cats who are still doing well while they are out of work. (Fantasy dressing: the characters know they are the reincarnation of some mythic hero, just as they know the villains are reincarnations of something or other.)
First, the murder hobos need weapons. No sweat: there's a cache of weapons in a nearby dungeon (well, the basement of a university). They can get to it through a sewer system, and they know about the sewer system because some guys hide there.
After killing only a few people (who must be the reincarnation of bad guys, too), they have ancient weapons befitting their original souls.
If you're playing as the murder hobos, well, this is all straightforward: the assault on the bad guy is an assault on his mansion.
Otherwise, you might be doing this as a kind of Call of Cthulhu thing, and the good guys are actually the local cops, who have been cast as "lawful evil" antipaladins or something.
Either way, you could play it in a way that makes the player characters (whichever they are) in the right. You just have to decide which way things are before you start.
You'd want a fantasy system that's easy to use and learn; you'd want the characters to be mid-level if you had a levelled system. Call of Cthulhu could do it. PIG's Iron Gauntlets might work. It's based on the Impresa system, and I think you can find guns and stuff as well as the fantasy equipment. Maybe CORPS, if you want something lethal. I think the equipment is around for BASH, too. Heck, you can probably do it with Unknown Armies or Over The Edge, too.