Have I told you I hate you?

I don't know why, but a part of my brain completely shuns certain people.
It's great. It usually happens to people my friends are friends with.
So I'm completely confused on what to do. I mean, if I'm not nice to the person,
then I don't feel right. So I HAVE to be nice.
And I can't tell my friends I don't like that one person,
because I'm the only one that doesn't like the person (or at least it seems that way).
It pisses me off knowing that I can't do anything against this feeling.
I want to say straight out to my friends that I don't like this person,
but I'm afraid of what will happen. Will they get mad at me,
and start shunning me. Or will they accept that fact and still hang out with that person?
Or will they start shunning that person as well. I REALLY DON'T WANT THAT TO HAPPEN.
I don't want to limit my friends to what friends they can have because that's like way too controlling,
and I don't want to be a controlling friend. It's limiting and jerk-ish.
I don't know what to do. This totally sucks.