Really old thread, I know. But I only just saw the movie, and my interpretation as to his motive in blotting out the instructions was to ensure that the ritual couldn't be stopped. He knew that she wouldn't reach the end without coming to terms with her own feelings (based on her past deceptions of both him and herself), so he made sure that things would come to a head and she would be forced to reckon with her own true feelings about forgiveness in order to meet her angel.