Although I called the rat evil, I later realised where he came from. I've been reading Rats: A Year with New York's Most Unwanted Inhabitants by Robert Sullivan on nicodemusrat's recommendation and it had a disturbing scene in which rat control officers were drawing blood from the hearts of live (unconscious) rats to check for infectious diseases. Although they kept upping the dosage of anaesthetic (enough to kill a cat, they said) some of these rats just didn't want to stay down. One struggled as he was being bled so they put him on the filthy floor and one of them stood on its tail to keep it still. I quote from the book:
Dan was not trembling but he did seem as if he were in shock. "I'm using my foot," he said, speaking in the present tense, as if watching a replay, "but the rat still gets up and pulls himself away." Dan was really shaking his head now. In fact, he was no longer telling me to keep rats in perspective. "Rats are incredible, they really are," he said. "I mean, there aren't many animals that can bleed like that and that can take it."Needless to say, they let that rat go on its way. I now understood why the rat in the dream glared at me so malevolently and would stab people in the heart. It was not evil; merely enraged and seeking revenge. We should be glad that rats do not have a vengeful spirit in them.