Ciro I wouldn't say is trustworthy as such, given that it seems he's at least a renegade although the lack of obvious mutations makes me believe he's either not full on Chaos Astartes or he's a very young one.
And yes, the Alpha Legion is just as mutated as the rest of them, they've been known to reveal mutations concealed beneath armor and cloaks as a shock/terror tactic against pious Imperials, anyway, where I was going with that was that while he may or may not be some kind of bastard, I'd rate him as the least likely of our comrades to put a knife in Vicenzo's back just because he's so secure in his position.
That position being the deadliest combatant, physically strongest, best tactician, and most likely to come through this all on his lonesome if he did just casually murder everyone else. He doesn't really need to play dominance games, everybody else needs to jockey for his favor (or at least his "I don't actively hate you.")
If anything, Vicenzo becoming KING OF THE CARRION THRONE would probably make him a more valuable asset/ally in Ciro's eyes, Borgia's already shown himself to be amendable to following his lead/susceptible to that literallt inhumanly shredded eight pack, why bother to personally command a feral abhuman tribe and if you can delegate a mortal you have a decent-ish rapport with to do it for you?
(As a side note, Ciro being a Renegade honestly doesn't help at all narrowing down his origin, any and all Marine Chapters have produced their fair share of Renegades, f'rex the Uriel Ventris novels have a Renegade Raven Guard playing a semi prominent role)
