“his hallucinatory belief that he had done it (he knew he hadn’t)”

From The Talented Mr. Ripley (1955), by Patricia Highsmith:

“Like the San Remo story. His stories were good because he imagined them intensely, so intensely that he came to believe them.
For a moment he heard his own voice saying: ‘… I stood there on the steps calling to her, thinking she’d come up any second, or even that she might be playing a trick on me… But I wasn’t sure she’d hurt herself, and she’d been in such good humour standing there a moment before..’ He tensed himself. It was like a phonograph playing in his head, a little drama taking place right in the living room that he was unable to stop. He could see himself with the Italian police and Mr Greenleaf by the big doors that opened to the front hall. He could see and hear himself talking earnestly. And being believed.
But what seemed to terrify him was not the dialogue or his hallucinatory belief that he had done it (he knew he hadn’t), but the memory of himself standing in front of Marge with the shoe in his hand, imagining all this in a cool, methodical way. And the fact that he had done it twice before. Those two other times were facts, not imagination. He could say he hadn’t wanted to do them, but he had done them. He didn’t want to be a murderer. Sometimes he could absolutely forget that he had murdered, he realized. But sometimes – like now – he couldn’t. He had surely forgotten for a while tonight, when he had been thinking about the meaning of possessions, and why he liked to live in Europe.
He twisted on to his side, his feet drawn up on the sofa. He was sweating and shaking. What was happening to him? What had happened? Was he going to blurt out a lot of nonsense tomorrow when he saw Mr Greenleaf, about Marge falling into the canal, and his screaming for help and jumping in and not finding her? Even with Marge standing there with them, would he go beserk and spill the story out and betray himself as a maniac?
He had to face Mr Greenleaf with the rings tomorrow. He would have to repeat the story he had told to Marge. He would have to give it details to make it better. He began to invent. His mind steadied. He was imagining a Roman hotel room, Dickie and he standing there talking, and Dickie taking off both his rings and handing them to him. Dickie said: ‘It’s just as well you don’t tell anybody about this…’”