I wouldn't say that the Capulets (or the Montagues, for that matter) are complex characters. They are there merely to advance the plot of the play, and to cause the tragic events which occur in the end. We don't really know much about either of the families, except for the fact that they have always had a particular feud and are therefore sworn enemies. They do act as a villain in the play, given that it is basically because of them that their children die because they weren't allowed to love each other.