That terrible hunger only went over the edge the moment Isiah murdered all those mages, he couldn't just ignore the fact that Ciel was going to be hurt by them and he run right away to save him .
He was more powerful than them, he sensed their divine energy from afar and knew where they were even before entering the institution.
Since he became capable of functioning with the diabolic core of magic, his senses got enhanced which helped a lot to find Ciel's place .
The first two he ran into were just wandering in the corridors searching for other people there.
Since they lack divine energy, they were unable to even detect that he wasn't just a normal mage and that he was different from them.
