At the time, I thought that Will Smith was justified in slapping Chris Rock for making comments about Will Smith's wife Jada Pincket Smith (who was/is dealing with hair loss). I thought he was justified in defending her honor (and perhaps his own be extension).
Looking back on it now, Smith's response wasn't warranted. They were just words, and had Smith left things alone the joke would have made Chris Rock look bad anyway. Instead he got upset with a joke, and let things go too far.