Pure speculation here, but given the fact that we haven't seen anyone else go full on "zombie/vampire" mode, I'm going to assume it has to do with the fact that Haruto was "killed" by L-Elf at the beginning of this anime. That's really the main difference between Haruto and the other 3 current Valrave Pilots.
Thus, it could be very likely that "hungry" means in order to sustain that hugely damaged body, Haruto needs to go zombie mode.
Thoughts. Comment!
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