Well, if the wish to be granted is to exchange the current best friend for 10 new best friends, I don't think they are supposed to, or even capable of betraying you. But then, a best friend relationship is not always mutual. Just because one party believes the other to be his/her best friend does not necessitate the other party to feel the same. I assume that having 10 new best friends means that you gain 10 people that you consider to be your best friends (I still have trouble with this phrasing, but I will accept it for the sake of argument), but they might hate your guts, and kill you for a selfish reason as Kraco proposes.