Holmes proves that the murder victim, John Straker is not,in fact, a victim of murder,but a villain with vile and sinister motives.
John Straker, who is also William Derbyshire,was leading a double life, including having a woman other than his wife, whoenjoyed expensive things and luxuries Mrs. Straker did not. Straker was, infact, a scoundrel, and his murder was nothing more than an act of self-defense.
Holmes can explain everything, it was Silver Blaze who killed John Straker. Despite being trusted, Straker had plotted against his employer, and had sought to make Silver Blaze lame by cutting him with the cataract knife, something he had practiced on sheep beforehand.