My primary guess as to "how this end in blood painting the walls and a mad demon-prince wreaking havoc beyond all control" is, well, what happens if one of his allies gets framed as a traitor, or gets killed by someone the sorcerer would rather stayed un-flayed for at least a few months longer?
I'd run it as once you're in Vicero's in-group, you're in, and he's going to shower you with all the world's bounty... except he's good at banditry and little else, so he'll be looting storehouses and kidnapping skilled craftsman to keep his friends flush with gifts, completely oblivious to the sociopolitical fallout because he's used to an environment where there are effectively no consequences for solving everything with theft & murder.
Also, he's completely sociopathic toward anyone who isn't one of his friends, and his only role model is the biggest bitch in Hell, so any "friends" he makes in Creation are going to see some pretty unpleasant shit happen to servants, underlings, and passers-by at his hands.[1]
Essentially, make it so there's a disturbing juxtaposition of the best and worst of Vicero's qualities, so that many mortals would come to find his presence deeply unpleasant and stressful. Once they see him put his fist through a concubine's skull for complaining about having to service one of his "friends", it's hard for them to forget.
[1] A good part of my inspiration is a section of a very good (if deranged) series: essentially, a time traveler from the 26th century ends up back in Victorian England, badly traumatized and in desperate need of refuge. He happens to meet Henry Beresford, 3rd Marquess of Waterford, who is captivated by the strange, well-spoken man who appeared out of mid-air wearing a strange mechanical suit and requested shelter. The time traveler spends many days at the Marquess' country estate, secluded in its back rooms and gradually adapting to the era he's trapped in by having conversations with his host, who he comes to be quite fond of. Beresford is genial, inquisitive (but not overbearing), and gives the traveler an anchor to cling to.
Then, one night, he hears noises down the hall, and walks in on Beresford, naked, taking a riding crop to a screaming, terrified woman - and when he's spotted, Beresford just winks at him and says "sorry about the whore, I'll shut her up forthwith. You're free to have her once I'm done, though, old boy!"
Because Beresford is a horrific human being, and the only reason he spared the time traveler said horrificness is that the traveler's manners and nature made the Marquess mentally classify him as an equal. Lessers get no such preferential treatment.