Antagonists ("Bad Guys") are fun to write about, for me at least. They can't be unpredictable though, once you have a picture of them set in your mind you can't change it, and having them also give readers a laugh can be quite interesting, although sometimes not the best thing to do. Antagonists can't suddenly change attitudes, if you do have them change, it should take a while for them to switch attitudes. If it changes to suddenly or to frequently it can get a little ridiculous. Often the antagonist is what spices up the novel or writing I am working on. They start to fill in the blanks where they were needed in writings for me, and whenever I have achieved a good antagonist I am proud of it.