The “empty” treasure box = Mary

So look at this description of ACD’s original story, The Sign of Four :

“tSoF is all about a group of four men who stole treasure from a city called Agra. One of these men confesses to Sherlock saying the treasure brought nothing but “a curse yet upon a man who owned it” bringing “murder… fear and guilt… slavery for life.” Mary’s dad dies as a result of the treasure and when recovered, it rightfully belonged to her and Major Sholto. After a lengthy boat chase along the Thames, the treasure is recovered and John (who at this point is crushing hard on Mary) goes to return it himself. John secretly wishes the treasure wasn’t hers because he wants to marry her and thinks that if he marries her after she’s suddenly become rich, people will think he’s a gold digger. Well, he brings it to her in this beautifully immense treasure chest. They open up the case and, to John’s delight, it’s completely empty. Johnathan Small had thrown it’s contents into the river. On discovering this, John proposes to Mary and all is well.”  (x)  

Now in HLV, Mary hands John a flash drive with what she says are her initials: A.G.R.A. The flash drive is supposed to contain the ‘truth’ about her past assassin work, which, if John reads, he’ll never be able to forgive. John doesn’t read it, presumably. He throws it on the fire at Christmas and forgives Mary. (which may or may not have been sincere, but is not the point of this mini meta).

I’ve seen fandom say that the flash drive was empty, because in ACD the original AGRA treasure box was empty. But what if MARY HERSELF is the ‘treasure’?

John married her. She was supposed to be this beautiful, fulfilling thing that gave him a normal life, one without risk of the type that SjHerlock brought. Sherlock broke John’s heart to smithereens when he fell, and John can’t take that again. Mary was the “safe” choice, the lovely treasure that would make him a rich man metaphorically speaking.

But that promise is an empty one because Mary herself is an empty treasure box. She has no heart or soul. Instead, she is a “curse upon the man who owns it” (i.e. John), and “brings murder, fear, and guilt… slavery for life”. Doesn’t that sound like what Mary, in her manipulative, dangerous, sociopathic assassin guise, indeed brings to John?

Yeah, I think Mary herself is the AGRA treasure box–the promise of a great treasure that turns out to be a curse and, ultimately, a completely empty promise. 

“Why is she like that? She wasn’t supposed to be like that.” Mary was supposed to be the beautiful, nice, sweet, SAFE choice. The heteronormative treasure. And in this moment John realizes she’s not that at all.