"People like Magnussen should be killed. That’s why there are people like me."

“People like Magnussen should be killed. That’s why there are people like me.”

-

“Mary Morstan”, Sherlock, His Last Vow

Right. I love how Mary tries to paint herself as some altruistic, ridding-the-world-of-bad-guys-one-at-a-time sort of person - a vigilante for justice, if you will - when what she actually is is a killer for hire. Mary kills for money, not for principle, and furthermore, that’s beside the point: the scene wherein the audience finds Mary dressed like an assassin, threatening Magnussen at gunpoint after having physically assaulted two of his staff, one of whom was her MAID OF HONOUR, has nothing to do with this. 

Mary didn’t go there to kill Magnussen, necessarily. Mary didn’t go there because Charles Magnussen was a horrible person who profited from blackmailing people over their secrets and histories. Nothing of the sort: Mary went there out of pure self-interest. It wasn’t a job; Mary was there to force Magnussen to hand over any incriminating material he had on her. Mary went there to keep herself out of prison, and to keep John from having the agency to make a decision for himself over whether or not he wanted to be married to her. She was willing to do anything to keep John from making an informed decision, up to and including killing his best friend. If Sherlock had died, Mary never would have told John that it was she who had killed him. She would have let him mourn, and been content to be the shoulder on which he cried, just as he always had before. 

Let’s not paint Mary as some sort of selfless agent of justice. She’s no Batman. She’s in this for one person and one person only: herself. This isn’t about what’s best for John, best for the world - nope. This is about Mary and Mary alone.