That's a long debated topic on the nature of evil. Is it the wickedness of the act or the extent of the damage? For instance, Hitler is widely regarded as the most evil man in history, but Stalin's body count was several times higher, and Genghis Khan trumps both with a whopping 60 million. The difference, I suppose, is Stalin went after political opponents - real and imagined - while Hitler went after people just for being who they were.
There's also active malice vs. mere disregard for life. Craster didn't do anything he did for the joy of hurting people, but simply because the results of such suited his fancy. Joffrey liked hurting people, liked it a lot. But you could make the argument that since Joffrey was satisfying an urge he's slightly less evil, whereas Craster felt no urge to commit evil, he just did it out of convenience.
It's a lot to throw around, and 3000 years of philosophy hasn't resolved it. But I'm sure we will on this thread!