Change of rules

The more Lucien spoke, the more Vam stared dumbly at him. 

Wait, wait, human souls? So they are all dead? Vam's gaping mouth begins to get bigger. 

Holy shit.

He can't imagine that violent human with a scar is actually a soul. He just used his skill to tackle down that waiter. 

While Vam was still having trouble accepting the truth, Aiden gazed at Lucien's absent minded eyes and answered his question.

"Because the young man with tattoos is the ghost. Naturally, monsters are at the bottom ranks in the Blood Tower bosses.  Their position in hierarchy is at the lowest, that is why they were looked down upon by the ghost bosses in the middle floors. Ghosts can eat the monsters' soul, energy and power but monsters are forbidden to devour ghost's flesh." Aiden took the time to explain although he got the feeling Lucien already realized these facts.