With the brow of a philosopher above and the jaw of a sensualist below, the man must have started with great capacities for good or for evil. But one could not look upon his cruel blue eyes [...] and the threatening, deep-lined brow without reading Nature's plainest danger-signals. (Empty House.1.68)
Some scientists in this period thought that you could tell a lot about a person's personality, intelligence, etc. through certain physical features. Watson here slightly romanticizes this scientific view and assesses the personality traits indicated by a criminal's physical features.
"There are some trees, Watson, which grow to a certain height, and then suddenly develop some unsightly eccentricity. You will see it often in humans." (Empty House.2.19)
Holmes's tree metaphor is in response to a discussion about what turns someone to a life of crime. For Holmes, a criminal nature can appear in a person late in life and rather unexpectedly.
"But he had not that supreme gift of the artist, the knowledge of when to stop. He wished to improve that which was already perfect - to draw the rope tighter yet round the neck of his unfortunate victim - and so he ruined all." (Norwood Builder.207)
It's interesting that Watson describes Holmes as an artist, and Holmes describes good criminals as potential artists as well. For Holmes, work can and should be done well whether it's crime or crime-solving. Holmes respects "artistic" ability, regardless of where it's put to use.
"If I shot the man and he had his shot at me, and there's no murder in that. But if you think I could have hurt that woman, then you don't know either me or her." (Dancing Men.185)
Abe Slaney takes a gunslinger idea of murder and seems to feel that it's only murder if the other guy doesn't get a chance to shoot back. This is one of many instances where a character quibbles with legal definitions of crime and criminals.
"He asked me if I would stand by the bargain. I said I would not. [...] I said I would have nothing to do with violence. So he went off cursing, like the foul-mouthed blackguard that he was." (Solitary Cyclist.146)
We get a good depiction of a falling out among a criminal gang here, which essentially boils down to issues of morality.
"I am convinced," said I, "that this Reuben Hayes knows all about it. A more self-evident villain I never saw."
"Oh! he impressed you in that way, did he?" (Priory School.1.153-4)
Watson's tendency to "read" people appears again here, as he suggests that the mean and rude Reuben Hayes must be a "villain" because he looks and acts like one. Watson is right actually, which is rather thematically telling.
"See here, mister," said he. "I make no complaint of being man-handled in this fashion, but I would have you call things by their right names. You say I murdered Peter Carey, I say I killed Peter Carey, and there's all the difference." (Black Peter.112)
The fact that Patrick Cairns gets upset about Holmes's language and not the fact that he tackled and handcuffed him earlier is hilarious. Cairns suggests an interesting distinction between a killer and a murderer here. There's a running idea in these stories that killing is unforgivable but that murder can be potentially justified.
"I've had to do with fifty murderers in my career, but the worst of them never gave me the repulsion which I have for this fellow. [...]"
"He is the king of all blackmailers." (Milverton.1.7-9)
Holmes views blackmail as one of the worst of all crimes, possibly because of the cruel and deliberate intention behind blackmail, as opposed to some of the murder cases he's worked.
"You know, Watson, I don't mind confessing to you that I have always had an idea that I would have made a highly efficient criminal. This is the chance of my lifetime in that direction." (Milverton.1.78)
The link between detectives and criminals is highlighted again here when Holmes considers how he could have been a good criminal. Criminal behavior comes down to a matter of choice here.
"You will ruin no more lives as you have ruined mine. You will wring no more hearts as you wrung mine. I will free the world of a poisonous thing. Take that, you hound - and that! - and that! - and that! - and that!"
She had drawn a little gleaming revolver, and empties barrel after barrel into Milverton's body, the muzzle within two feet of his shirt front. (Milverton.1.108-9)
In one of the book's more dramatic and violent scenes, we get a depiction of justifiable homicide in action as a wronged woman gets revenge against her blackmailer. Holmes defends this murder even though it was planned and deliberate.
"I have not seen a man who, if he turns his talents that way, was more calculated to fill the gap left by the illustrious Moriarty." (Three-Quarter.147)
Again, the criminals and detectives are linked together. Both groups are potentially united by talent and similar skills but are ultimately separated by choice.
"Do you think I was sorry? Not I! It was his life or mine, but far more than that, it was his life or hers, for how could I leave her in the power of his madman? That was how I killed him." (Three-Quarter.163)
This is one of the bolder confessions in the book, and one of the more unapologetic ones as well. It's interesting that Jack Crocker uses the word "kill" rather than murder here. How does this compare with some of the other confessions of killing/murder in these stories?
"A criminal who was capable of such a thought is a man whom I should be proud to do business with." (Priory School.151)
The use of the word "business" really highlights how Holmes views his detective work and his interactions with criminals, particularly skilled ones.