I think it's simply that vampires must have blood in order to live (and remain sane - not become beasts), and thus vampires that cannot replace live blood with the tablet are dangerous because there's no choice for them but to eventually succumb to their thirst and create a victim.

We have seen even the nobles surrounding Kaname have from time to time troubles suppressing their thirst, and thus it's a problem if blood is spilled in the school and the delicious odor spreads. It's quite inherent for vampires to desire blood, but I'd imagine the higher class they are the more they take care to remain in control.