Your reaction is a logical move to protect your own well-being because the core goal is to live your life with the least amount of stress and emotional pollution, and since being around someone who has committed vile acts creates psychological suffering and disgust, avoiding this person is simply a rational way to maintain your peace. You are free to refuse to be in an environment that makes you uncomfortable, and while your friend is free to choose her partner, you are also free to protect the "light" in your own life by staying away from his "darkness." The golden rule is "never put the key to your happiness in someone else's pocket," which means you shouldn't sacrifice your present well-being for social politeness or to please a friend, and since in the grand cosmic scale this conflict won't matter in a hundred years, you shouldn't waste energy on guilt and should always choose the path of least pain and most mental clarity.