#Expectations
Whether it's "evil" or "humanity", keep in mind that both is not an absolute guideline for every single action somebody takes. It's more like a drop of ink in a bottle of water. Just one little drop taints the whole bottle's contents.
A person can live a peaceful life, be nice and helpful for 50 years to thousands of people they meet, but if they rape and kill someone once, they are a vile beast. This is how our morality system works.
An SS prison guard (an example I think we can agree on as both inhumane and evil) will have photos of him with his guard buddies, wife and dogs, being a happy family at a barbecue. People that didn't suffer from his cruelty might tell how good a comrade he was and how helpful and cheerful.
Being inhumane and/or evil is not something you are every second of your life.
So don't expect players of inhumane or evil characters to be that way in every interaction and with everybody. Remember the ink. A single drop is enough.
#Solution
So you have this game and you actually want people to play according to their alignment/humanity score/empathy value or whatever it's called in-game?
Well, don't let them start out with anything but neutral. Let their actions speak. Make it easy to change that value and then have them change it through their actions. If the evil assassin wannabe only helps princesses find their lost kittens, he'll always stay the neutral rogue. If your vampire is only helping in hospitals doing a faster dialysis for sick people, he'll always be at humanity 10.
Just make sure that switching is easy. Remember, killing a single innocent is be enough to be branded "evil" for life in our world, don't make people do more evil things than normal just to be able to be "evil". Same goes for inhuman. You only need to be openly inhuman once in a while. All your neighbors will say "but he was such a fine young man, always helped me with my grocery bags", that's not enough for "humane", that still leaves plenty of space for inhumane.
#Possible Explanation
I have never been at a LARP, but at a table, evil or inhumane actions mostly target fantasies. People only existent in the minds of the players. It's easy to be evil then. Being evil or inhumane at a LARP might come at the expense of a real human being. Of his or her fun at this event, because it's not fun if one's character is betrayed or killed in the sleep or whatever heinous deed one might commit. So maybe inhumane or evil just don't work too well at LARPs, where you are always hurting a real persons fun or feelings.