The stated goal of Dungeons & Dragons – and by extension Pathfinder – is to be a role-playing game where the players play heroes. What it actually is, however, is a role-playing game about killing creatures and taking their stuff.
The disparity between what the game comports itself to be and what it actually is is usually bridged without too much difficulty. After all, just make sure that the creatures you’re killing and looting are evil monsters bent on wreaking some sort havoc, and you’ve pretty well solved the dilemma. (I won’t get into the issue of other play-styles beyond the usual “killage-and-pillage” here, since I’m painting in broad strokes.)
The above merger, however, starts to fall apart if the players or DM try to push things too far in either direction. A character who starts to really push the heroic angle can quickly derail things if he starts to, say, put too much value on the life of other sentient creatures. After all, contemporary heroes place a premium on all life, even that of their enemies – when’s the last time you saw Batman kill some thug and smile about it?
…okay, bad example there, but the broader idea still stands.
Likewise, the reverse is also true; PCs that become excessive in their use of deadly force can quickly slip away from any resemblance to a heroic archetype. We usually see this used as a gag in things like Knights of the Dinner Table and The Gamers: Dorkness Rising where the “heroes” are bumping off NPCs left and right with attitudes ranging from nonchalance to outright joy and amusement at their demise, usually to the frustration of their Game Master.

"So, we're all cool about that peasant I set on fire? Seriously, I still say he looked like a demon."
The above sentiment – that supposedly-heroic PCs who act like homicidal psychopaths are a perpetual joke of the genre – was about all the thought that I’d given the matter. After all, that was good for a joke or two, but that was it; you couldn’t really take that sort of game-style seriously in D&D/Pathfinder…it eschewed the goal of being heroic, and that’s not what the game is geared towards. ’nuff said.
I started to think differently, however, after a player in my Pathfinder game was telling me about the Dark Heresy game he played in. I know I’m butchering this (which seems somehow apropos), since there’s no aspect of Warhammer that I’m familiar with, but he was going on about his (home-brew) space marine character who functions as the fanatical inquisitor in an evil empire, dedicated to rooting out not only internal weakness and rebellion, but also fighting external threats such as aliens and daemons.

"Say 'Blood for the Blood God,' again. I dare you."
After hearing several stories about how his character, in a very Darth Vader-esque manner, killed minions of the Empire who were too incompetent or fearful, I commented on how his character sounded rather Lawful Evil. He nodded enthusiastically, and outlined that the entire Empire was pretty much Lawful Evil, and most of the enemies it fought were Chaotic Evil, with a few Good types out there somewhere.
This got me thinking…why don’t we have something like this for Pathfinder? I’m not talking about a bunch of supplements outlining new (and mostly unnecessary) rules for evil characters, or a grim and gothic new campaign setting where there are no “points of light” to be found, but rather…why not make an adventure, or even an adventure path, for evil (or at least non-good) characters?
Would that really be so hard to do? An adventure where the PCs are retained by a mysterious patron to invade a temple, steal a religious artifact, and deliver it back to him for a hefty sum of gold doesn’t really change that much if you make it a temple dedicated to the God of Good. How difficult is it to make “rescue the princess from this stronghold she’s being held in” instead be “Kidnap the princess from this stronghold where she lives”?
Not all adventures can be so easily re-skinned, of course, which is fine. The same way not all good adventures lend themselves to an evil adaptation, great adventures for evil characters would usually be original ones too. Imagine, for example, a quest for a higher-level group of evil characters where they’re retained to attack a small town and take its population into slavery, perhaps on the behalf of an evil cult that requires a large number of sacrifices.
Such a setup would make for a great sandbox-style adventure. What targets do you attack first to help knock out the town’s defenses? How do you round up the townsfolk without killing them (since you don’t get paid for the ones you don’t deliver alive)? Can you deal with the surprise attack by would-be rescuers as you’re marching them off to be sold? Throw in some situational mechanics – such as giving bonus XP based on taking key NPCs alive for valuable slaves, making an incentive for the PCs to pull their punches against the deadlier good guys – and you’ve got a great adventure on your hands, perfectly suited for evil characters.

Should Paizo's next adventure path be...evil?
Now, there are some issues that come up in evil games that you don’t have to deal with when playing good guys, it’s true. These are largely problems of intra-party fighting, and some people being uncomfortable with the actions of other PCs. These aren’t trivial concerns, since they can quickly ruin peoples’ enjoyment of the game, but it’s important to remember that these are issues regarding the group, not the game itself. If the players can act maturely, respect each other, and work things out like reasonable adults either beforehand or as problems arise, there’s no reason why an evil campaign can’t work as well as any other.
After all, if an evil campaign lets friends to get together and have a good time, then it’s serving the real goal of the game: for everyone to have fun.