Humanity + Ending
When you get to the last boss, "vukolak" or whatever his name is, you better have some humanity. If you have too much humanity (70+, I think), you don't get choices. You have to fight him. If you have humanity 40 - 55, you get choices and you can choose whatever. If your humanity is too low, than you only get to diablerize him and become evil. Tough break.
PS: The only part of the game that you can actually GAIN humanity, is when you talk to Prince Rundolf Brandl of Prague and keep asking about the sake of the kine slaves. The Prince gets pissed off and hits you but it's no problem, you don't get punished for it and get +10 humanity.