I've been thinking about adopting some kind of humanity rules for my Unisystem game.  I personally think the nWoD method is rather appropriate for Buffy.  There's even a good canonical example from the show  (spoilers below).  

In Season 3, Buffy and Faith accidentally kill a man.  He came out of the shadows, they thought he was a vampire, and Faith staked him.  Buffy was torn up with guilt about this, made worse by covering it up.  Faith honestly just didn't care.  It really did not bother her.  The guy got in the way, he got killed, not going to lose any sleep over it.  It is almost a perfect reflection of the WoD mechanics - Buffy made her Humanity check and is overcome with guilt.  Faith failed hers and started the spiral toward evil.

I'd like to have something like this.  I don't know if I'd have actual checks for it, but keeping tabs on where someone's Humanity is might be a good idea.  Perhaps I just judge whether they slide down the scale or not based on how they deal with what they've done.  For instance, in my current game I have a character that killed a couple of people.  They weren't good guys by any stretch, but he still chose to go for a kill when he could have let them walk away.  Figuring out how we'll deal with that should be an interesting time.

Any suggestions?

(Not having read nwod)  What does it get you to have high/low humanity?  There should be mechanical costs/benefits to sliding into evil if you're going to make it into a an actual rule; if not, it's best to leave it as an casual, unofficial thing.

Maybe certain classes of demons can't attack people with a very high level of humanity.  The devil can't touch anyone with a pure soul.  Other monsters might be able to detect goodness, especially really pure souls.  Some artifacts might be usable by only good or only evil people.

Quote from: Nicephorus(Not having read nwod)  What does it get you to have high/low humanity?  There should be mechanical costs/benefits to sliding into evil if you're going to make it into a an actual rule; if not, it's best to leave it as an casual, unofficial thing.

Maybe certain classes of demons can't attack people with a very high level of humanity.  The devil can't touch anyone with a pure soul.  Other monsters might be able to detect goodness, especially really pure souls.  Some artifacts might be usable by only good or only evil people.

Well, in nWoD as you slide down the humanity scale you pick up mental problems.  Probably wouldn't do that in Buffy.  I could see certain spells getting a discount if you have a high humanity.  Maybe provide a bonus to resisting some dark magics as well.  And the really evil stuff would cause a drop humanity maybe.  For a scale I'm thinking of something like

0 - Monster.  No soul and no human affection.  Adam, Angellus, and The Master would all go here.
1 - Evil.  Creatures at Humanity 1 may be human but be totally corrupted, or they could be demons/vampires that have some manner of affection.  Spike and Drusilla in Season 2 would belong here, though obviously on the low end, due to their love and affection for each other.    Mayor Wilkins would belong here, due to his affection for Faith, as would chaos worshipping sorcerer Ethan Rayne.  Speaking of Faith, she would belong here after going to work for the Mayor.
2 - Human.  Most humans go here.  They care about their friends and faimly and aren't likely to go about murdering people.  They may have a bit of a dark side or mean streak, but generally keep it under control.  Giles would belong here, as would Faith before her betrayl.
3 - Principled.  These are the folks that likely have that 'honorable' disadvantage.  They consider others before themselves, even complete strangers.  They would never think of killing another human, and even less severe transgression inspire bouts of guilt.  Buffy and most of the Scooby gang would be here.  The very fact that they choose to fight against the darkness rather than look after their own necks means they care about humanity.
4 - Moral.  These characters go above and beyond.  They not only risk themselves, but may focus their entire livelihood around protecting others.  Riley Finn early in Season 4 may qualify, but characters this staunch are rare.  
5 - Paragon.  These characters are utterly selfless and have virtually no temptations toward evil.  They don't appear very often because, well, they aren't very interesting.

On a 0-5 scale, I'd have the typical human be 3, not just one step above the average vampire.  Maybe have 2 be dirtbag human, with some morals but not many.  Principled and Moral don't seem different enough.  I'd make moral a bit more paragonish and be 5.

You may want to check out the Madness Meter rules in Unknown Armies.  There are five different categories that the MM checks against: Violence, the Unnatural, Helplessness, Isolation, and Self.  If you fail a stress test, you gain a Failed notch in that category, with the more you accumulate, the more your character is affected by that particular trauma; if you accumulate 5 failed checks, time to get some therapy.

OTOH, if you succeed against a strees test, you gain you become more Hardened against that particular type of stress.  At low levels this isn't necessarily bad (you don't freak out upon getting into a fight after a few Hardened notches in Violence, for example), but becoming too Hardened (ten notches) isn't a good thing, as that means you are losing touch with the emotions that make you human (i.e. Faith's descent into becoing a sociopath).

Actually, I recommend picking up the Unknown Armies corebook in general.  It's an excellent game, and has lots of wierdness that can be ported into a (rather dark) Buffy campaign.