Origin letter reading for Quilty in the opening scene
When Humbert force to Quilty to read a letter before kill him, for Humbert that letter is the "Sentence of death", the letter says:
"Because you took advantage of a sinner"
"Because you took advantage..."
"Because you took..."
"Because you took advantage of my disadvantage"
What is the origin of that text?
Why Humbert want that Quilty read that letter?
Why that letter is his "setence of death"?