I hadn't been particularly enjoying it for a while - I was finding things that should have been exciting really tedious, and was growing a little weary of the endless violence and main characters' plans being foiled - and then something happened that was so gratuitously nasty and unpleasant that I realised I didn't want to know any more about any of these characters. I've never DNF-ed a book because of a single event, I'm not particularly easily shocked, but it was a combination of the deeply awful nature of that event and the fact that it crystallised for me the fact that I just didn't care enough anymore about the story to put myself through any more of that nastiness.