It's been a while since I read this, and it was one of those utterly forgettable paperback covers. The main character is an immortal person on a generation ship that is originally scouting the stars, then ends up headed towards earth to repopulate it. He gets his memory wiped every 20 odd years, and doesn't know he's immortal til the end of the book.
He kills the captain of the ship (who was also immortal, I believe), and takes command of the ship, steering it back to earth. There was sort of a side plot with some old lady who has been breeding the ship dwellers to be nonviolent by manipulating the breeding lottery. Any help finding this would be very appreciated!