I don't think violence matter all that much to make it from PG-13 to Rated R, at least not the way it's done in the Harry Potter series I don't know though. It would suck for the kids if it was Rated R but her last couple books really seem like that are geared for late teens and adults with all the darkness.
I'm also sad to see Tonk's with Lupin, she's supposed to be with me! If you haven't noticed she's on my Avatar!