Why don't you like the theory Donald Love is Mr_Nasty?
Personally to me at least it makes sense. I'll list why this theory is so believable. But I'm definitely interested in those who are opposed to the idea!
Donald Love is from GTA III, the same GTA that that explicitly mentions Carcer City events. More so than the Airport terminal in San Adreas, or even Michael's mention of his first heist in the city. In my opinion Donald appears in the game that most notably has a bigger connection to Manhunt vs any other game. (An exception being GTA V online's character masks, Piggsy tattoos, etc)
Donald owns Love Media. Stated to control approximately 900 radio stations, 300 television stations, 4 networks, and 3 satellites nationwide. Also newspapers. Showing, he's obviously rich af, but also well involved DEEP with entertainment. It's literally his job, why wouldn't it plausible for him to also be running a snuff ring? He is known to have changed laws, been involved in slum clearance, and opened casinos across the United States, in the process becoming a "legend" according to a Liberty Tree report. Even if he didn't dabble with Starkweather he most certainly knows of him. To me it makes perfect sense for him to own Valiant video.
His quotes and the cannibalism alone tells me all I really need to know too. "...you can be very loyal for the right price... But groups of men get greedy." And, "I have some more delicacies about to be served... the liberals would have a field day, of course, but how little they know of life's real pleasures!" This man is already wicked as shit and when you take in consideration that he owns buildings, even an import garage—something just screams at me. Why wouldn't it be him? He is also known to throw morgue (diddy) parties, as revealed in both GTA III and GTA Liberty City Stories, even having Toni Cipriani steal the corpses of Avery Carrington and Ned Burner for him to eat.
From the wiki: Love is portrayed as a total hedonist – he believes that everything is for the people, especially for him, showing some arrogance and egocentrism. He is also known to have a very sociopathic personality, showing no qualms when it comes to eating other people or hiring Toni to destroy an entire neighborhood, Fort Staunton, for his own financial gain. The only thing that matters to him is his own enjoyment.
"Only thing that matters is his own enjoyment." Mr Nasty, in my honest opinion, is 100% this dude. Suuuure Mr Nasty is well anonymous, but anonymity can totally be bought with money and power. However, lots of people though dislike this theory, and I'm interested in your opinions.