If you hear that a friend of yours has done something terrible, please go and talk to them and find out if the rumours are true _before_ judging them or joining in the general condemnation.
Similarly, if they do something which you find hurtful, please talk to them and say something along the lines of "When you did X, I felt Y. Did you mean to hurt me in that way?", rather than assuming that they were trying to stab you repeatedly with a sharp knife.
Once you have ascertained that your erstwhile friend really _does_ strangle puppies in their spare time, then feel free to cast them forever more from your social circle, keep the DVDs you borrowed from them, and post embarassing photos of them on Facebook. But if you don't do your fact-checking before your ostracising then you're not being anything like a good friend. You are, in fact, being a jerk.
All the best,
(No, this is not aimed at any one specific person - there are three separate people I know who are having issues around this _right now_, and I've seen it happen innumerable times.)