Elaine was a planner through and through. She had worn gloves the entire time she had set up the aerosol can and made sure there were no security cameras around that would catch her in the act on film. She had also secured an alibi in case things went south or if she was successful in killing him. A human corpse was a lot harder to wash her hands of than a wolf one.
She stayed around long enough to see the police walk up to Devon's motel then got into her car and left to grab something to eat, a part of her alibi. Elaine may have left him for now but it wasn't over by a long shot. She knew what he was and she felt it her duty to find him again and kill him. He wasn't a man, he was a monster that would hurt someone sooner or later if he hadn't already done so. The fact that he now knew what she was put a wrench in her in her plans but it wasn't the first time a mongrel had been aware of her true identity and most winded up dead.