Honour amongst criminals
"'I'd a partner,' said he, 'a rare good man, as true as a stock to a barrel' He's got the dibbs, he has, and where do you think he is at this moment? Why he's the chaplain of this ship -- the chaplain, no less!"
Not only is it ironic that a criminal intent on the murder of his fellow shipmates being described as "a rare good man", but he has taken on the role of chaplain on the ship. A chaplain is a person who performs religious duties in a military group, yet this man has money to bribe others and reason to kill those who wont take the bribe.
The death of the trainer
"We may have the explanation as to why John Straker wished to take the horse out on to the moor. So spirited a creature would have certainly roused the soundest of sleepers when it felt the prick of a knife"
John Straker, intending to injure his employer's horse in order to place bets against it, is killed by that very horse. In attempting to take a knife to it in what he though would be an unnoticed crime, he was kicked by the horse, bashing his head in and giving himself a gash across the leg with his own knife.
Sherlock personal system of law
“It’s every man’s business to see justice done.” vs "I follow my own methods, and tell as much or as little as I chose. that is the advantage of being unofficial"
Of the two quotes, the former suggests a man who has taken the role of crime solver out of moral obligation, however the later suggests that he follow his own personal moral code. Although the rival trainer that hid the missing horse has broken the law, Sherlock has no wish to see him punished for it, perhaps because his involvement could see him charged with murder wrongfully, or perhaps just because Sherlock knows the man has been scared by him into putting it right.
cocaine
“Save for the occasional use of cocaine he had no vices, and he only turned to the drug as a protest against the monotony of existence when cases were scanty and the papers uninteresting.”
It seems a strange irony that a man such as Sherlock Holmes would use a mind altering drug, as it is his mind that he values above all else. This is stranger still that his friend, a doctor in his own right, would be so used to this as to mention it of hand with no protestation.
Hinsight
Often when Sherlock makes his deductions he sees and interprets things that others overlook, his clever nature allowing him to see meaning in the mundane. Although the observations are simple, many cannot see them and lack Sherlock's ability to understand them until they are explained. It is ironic, however, that once they are explained in the simplest way, only then can many others see the clues that Sherlock has obtained, yet they believe Sherlock cannot be all that clever because of the simple reasoning he gives.
