been thinking about this and why it would be the case
now, it's entirely possible that Black Death is lying, though I find it more likely that he'd be repeating a lie from Black Doom
he gives me the vibe of loyal follower who's bought into the leader's lies, who actually believes what Doom uses as excuses and manipulation
that all being said, my only idea is that Black Arms thrive off of whatever living beings put out when scared like adrenaline
there are species that keep their prey alive longer but that's more to keep it edible longer, like a preservative; it's not a nutritional thing as far as I know
maybe they're partly vampiric and require the blood
though, since they also eat fruit, I do find it more likely that Black Doom made it up as a "this is why we have to do this" excuse
esp considering how Black Doom tried to frame it as justice or salvation in an attempt to get Shadow to join him