That theory, that Holmes and Watson meant to kill Milverton, is nothing new in the wonderful world of Sherlockianism--I've seen it a number of times before. (Before the BBC series was even a gleam in Moftiss's eye, that is.)

Sure, Mofftiss do love to play with Sherlockians' knowledge of the stories. They know readers of "Charles Augustus Milverton" will expect Mrs. Smallwood to kill CAM, so they have Sherlock think the same by having her and Mary wear the same perfume, for example--just as, in ASiP, "RACHE" was German for revenge (and not a woman's name) in the ACD story, and the exact opposite in the show!

Still, their comment seems to indicate that they see Sherlock and John from the canon as characters who have no problems with committing murder if this suits their plans. And I am surprised why Moftiss see them that way. As I explained above, this is in very harsh contrast with the manner in which they are characterised in Doyle´s stories.

But there´s a difference between not caring if the criminal is caught because the victim was an awful human being (as for example, in the case of "Devil´s Foot") and planning and performing cold-blooded murders yourself.

Sherlock Holmes was not an "avenger", but also not Batman or Dexter. And this sounds very OOC for him.

Well, as I said in the first paragraph of the post that started this thread, there were many theories that Sherlock could have killed Milverton, but in self-defence. He already found a way to burgle Milverton´s safe and without the compromising letters (which were a real bane in Victorian society), the blackmailer would be helpless to do any actual damage. So there was no genuine need for Sherlock to shoot Milverton in the canon (unlike in BBC version, where he was pressed to do it by the threat to John and his family).

In "Sign of Four" we read this confession of an accomplice to murder:

My heart softened to him, but again the thought of his treasure turned me hard and bitter. I cast my firelock between his legs as he raced past, and he rolled twice over like a shot rabbit. Ere he could stagger to his feet the Sikh was upon him, and buried his knife twice in his side. The man never uttered moan nor moved muscle, but lay were he had fallen. I think myself that he may have broken his neck with the fall. You see, gentlemen, that I am keeping my promise. I am telling you every work of the business just exactly as it happened, whether it is in my favor or not.

And this is John´s reaction to what he had heard:

He stopped, and held out his manacled hands for the whiskey-and-water which Holmes had brewed for him. For myself, I confess that I had now conceived the utmost horror of the man, not only for this cold-blooded business in which he had been concerned, but even more for the somewhat flippant and careless way in which he narrated it. Whatever punishment was in store for him, I felt that he might expect no sympathy from me. Sherlock Holmes and Jones sat with their hands upon their knees, deeply interested in the story, but with the same disgust written upon their faces.

And those men, so disgusted with the story about how the person cold-bloodedly helped to murder something, should be all "Right, we are goint to have to go and kill him, aren´t we?" as Moftiss put it? I doubt it.
 

But wouldn´t damage canonical Watson´s outlook on his friend if he witnessed him killing somebody in a murder planned beforehand? Would he be still able to write such gushing praises of him? It seems unlikely to me...