Looking for a book, or series of books, that had a character called a Vail or Vale that was basically a tracker and would kill the people tracked. When the person tracked was killed, the Vail started to assume their characteristics and started to realize that what they were doing was wrong. Tracked the main character through space.
In the Rings of the Master series by Jack Chalker the main characters have a nemesis called a Val (named after the flying dutchman IIRC) that pursues them. It is an agent of the all powerful computer created to save the human race (only a slight spoiler, please excuse me). This sounds like the story you are recalling