Upon returning to the mansion to rest for the night, and to avoid the ever watching eye of the local Horde King and his agents embedded throughout the city. Zane quickly secluded himself in his room, where he devoured the remainder of the hearts. Or those that he had allocated for himself.
Awakened hearts were valuable resources for gaining vast amounts of experience over short periods of time. Without them he would have to rely on tens of thousands of blood crystals to get the job done for a single level gained.
Which while he could easily reap such a bounty. Such a large massacre of Victor's forces would immediately reveal Zane's identity. And that was something he wished to avoid at all costs. Because of this, he consumed enough of the Awakened hearts to instantly boost his level from 30 to 40.