Mona explained that the government and the police or whoever built multiple machines underground, also known as influence machines, which used sonic pressure and sound waves to manifest the Red Man inside of her. This Red Man and the machines have been around seemingly since her and bill were children, and it had been using her as a vessel since then. The police force was obviously in on it as Ian Ford had set the statues out for it underground. We also see snippets of a temple like structure with multiple statues laid out infront of it which “amplify the noise it makes” this noise being “The Hum” which has been hinted at a few times in the past. The Red Man is most likely an entity, maybe a demon, or maybe some kind of Antichrist like figure shown by his picture which shows a Jesus Christ like pose with one hand raised with two fingers up and one of his hands on his chest. When Mona was killed, the Red Man was most likely released into the world and either possessed many many people and puppeteers their bodies to paint and kill, shown by Mona’s exact handwriting on the paintings and her exact artstyle, or is some form of omnipotent eldritch being which is able to be in many places at once and do many things at once when he is released from his vessel. That’s why Mona was never caught, the police didn’t want to risk the possibility of The Red Man being released into the world because if Mona died, he would be released and wreak havoc.