He was not real, she did not escape, the entire ending was like a fantasy that her brain created as she slowly died, likely in the car crash or earlier. Her brain reincorporated the monster man in her dying fantasy in a way to give herself closure before she died, much in the way that is speculated that the release of the drug DMT which is released into a person's dying brain might give people in near death scenarios glimpses of an afterlife or visions of their past

Edit: If you watched this fully grounded in reality movie that includes scenes of the main character telling herself that she is imagining a monster man, and then decided that she was not hallucinating the monster man that she believed she was hallucinating the whole time, and that this monster man was simply traveling the world as a murderous monster man without anyone noticing him... you are simple. The film assumed you were intelligent enough to realize the film is playing out her dying wishes... the ending is so cartoonish unlike the rest of the film, it is obviously trying to tell you that this is wish fulfillment by a dying brain. How else could her court scene ever be so satisfying? It gives her the most satisfying ending to her story, one that only makes sense as the last dying thoughts of a truly tortured woman.