I also think that Alfred's reluctance of not revealing the letter to Bruce highlights the theme of morality and it's the complexity.
Alfred, as good and noble as is, made a choice and really, this creates a grey area of our choices in life and it's consequences. Like how Batman chose to save Rachel rather than Harvey despite him being an important public official ad possibly a heir to Gotham's crusade when Bruce hangs up the cowl.
But Bruce chose Rachel because he loved her since childhood. There is no right answer. And that is how life is and operates.