The World of Darkness is an urban fantasy game set in a dark fantasy style where players reprise the roles of supernatural creatures whose very presence makes an already dark world an even darker place. The mundane world and the people in it are petty and selfish partly because humanity is its own form of monster and partly because its better for the monsters if humans behave that way. While it does have its own form of Other (the Belials Brood, the Thrill Kill Clubs), the horror they evoke is more along the lines of "But for the grace of God, there goes I". It's a deeply personal form of horror dressed up with false empowerment. Sure, you can run really fast and bench press a lot but your unnatural anger, strange manner of behaviour, and bizarre sense of right and wrong alienate you from everyone else around you. That alienation can also lead you to do really nasty things and such mystical powers, while not inherently good or evil, tend to hurt more often that heal.
In other words, what empowers you can lead you to disempower and destroy the very things you want to cherish.
Call of Cthulhu is a straight horror game where humanity and the mundane scientific reality may not be inherently good but it's the best we have. Humans may be more like children of the cosmos toddling around our playpen rather than masters of our own destiny, but we're generally well-meaning and try to do the right thing even as we fail. There is a similarity with the World of Darkness in that we have no friendly parental figure in the form of a God. In fact, the Gods ignore us but humanity act like neglected children and some seek them out anyway. There is goodness, or at least a wholesomeness, in being human. The goodness isn't something that can be held up against a stricture nor measured on a human's soul. It is an entirely artificial human conceit. But that doesn't mean it's not important or it doesn't matter to those individuals involved in it. There is nothing good or redeemable in the supernatural elements or creatures upon this world and humans who attempt to deal with such entities or learn magical spells will slowly find themselves becoming tainted and going slowly mad.
In other words, what empowers you can lead you to disempower and destroy the very things you hold dear.
As you can see, there are similarities but the differences are immense and the change in focus profound. If you dial down the mundane cruelties of the World of Darkness and make the world a bit more ordinary, you can show off the extraordinary cruelties of the supernatural denizens. The trouble is that most people play WoD for the chance to be a supernatural creature, often for a sense of empowerment, as while there are creatures even the monsters fear at least your characters don't have much to fear from a random mugger. And let's face it, it's the very real mugger that most people actually fear rather than vampires or werewolves.
Playing a monster in a game that's true to the Cthulhu Mythos (and, to a lesser extent, truer to the harsher side of WoD), leaves a character becoming slowly possessed and consumed by their supernatural corruption until there is nothing left of the original person. While Death Rages and Frenzies do have an impact on the game and can whittle away your character's morality, that's nothing compared to knowing with absolute certainty that your future is as a sociopath or a violent warmonger or an insanely whimsical and cruel creature of beauty and terror.
In my James Tyler campaign, I have a player who enjoys being the Good Guy. He admits that he's not really a horror player though I've found a way to get him to loosen up and start to enjoy the horror through a Supernatural-style 'monster of the week' episodic campaign. I've let him play at being a more human character, despite being a vampire, but that doesn't mean that I haven't dropped a few hints as to the wrongness of the kindred condition.
True, I like the whole "sympathy for the monster" routine because it keeps things from getting simple but a "Monster of the Week" campaign kinda craves real Bad Guys. The way I've dealt with this is to make humanity (and human goodness) an artificial conceit as it is in Call of Cthulhu but without it a character loses themselves to madness and cruelty. This kind of fits in with Morality which is pretty artificial itself though I have detached Morality from strict 'Thou Shalt Nots' and made it more a number of times you can do the 'Wrong Thing' before you completely lose yourself. Stealing a serial killer's knife isn't going to lose you anything but stealing $20 out of your mate's wallet when they trusted you enough to leave it out most assuredly is.
Since I'm running a vampire campaign I can put a lot more interest in the whole corruption by the supernatural deal. James has earned a few coils which helps him throw off the curse just a little but those coils are simply a delaying tactic. As he ages, I will slowly reinforce that simply being a monster is corruptive, not just to yourself but to other people. He will begin to struggle to see those he hasn't attached to through his coils as objects (namely tasty or threatening ones) and will have to keep pulling himself up on that. As a human he was a heterosexual but as a Daeva he will slowly lose that standpoint and start seeing men and women more equally.
I know my player and he'll be okay with this. In return, because a roleplaying game is always about concessions, I will hold out the opportunity to cure himself before it progresses too far. The cure won't be an easy thing and it will have temptations of its own but should still be interesting. The evidence of his own growing inhumanity should also work to galvanise him and be like a more optimistic version of the various Call of Cthulhu taints (where the end result is that it is incurable and you fail).
Anyway, that's enough musing for now. What do you think? Can the World of Darkness be blended (somewhat) with Call of Cthulhu or are the two doomed to forever run counter to each other? You can read more about my Tatters of the King (call of cthulhu campaign) run with a vampire character over here.