What was behind the disguise of Stapleton, and why he did use it in Hound of the Baskervilles?
Holmes believes that Stapleton disguised himself behind a bushy beard so that he could accost Sir Henry Baskerville in London undetected. Holmes thinks that Stapleton had hoped to do away with Sir Henry in London, before he got down to Devonshire at all. Unfortunately for Stapleton, Sir Henry received a warning that his life was in danger from Stapleton's wife. Sir Henry immediately solicited the help of Sherlock Holmes, and when Stapleton discovered this, he realized that he would have to carry out his plan to eliminate Sir Henry on the moor by the Baskerville estate, and, with this in mind, contented himself with stealing one of Sir Henry's boots, to be used as a means of setting the hound on the unsuspecting gentleman's track.
Stapleton is third in line to inherit the Baskerville fortune. The son of a shady Baskerville relative, his existence is unknown to the inhabitants of the Baskerville estate. When he becomes aware of his own identity in relation to the fortune, Stapleton, who has used a number of aliases in a life filled with disreputable and failed undertakings, travels to Devonshire with the objective of neutralizing the Baskerville heirs who stand between him and the inheritance. Using the ingenious but sinister method of scaring Sir Charles Baskerville to death with a demonic-looking hound meant to embody the legendary Hound of the Baskervilles, Stapleton then dons the disguise with the bushy beard in hopes of intercepting Sir Henry, the only remaining obstacle between him and great riches, and eliminating him in London. His plans, however, are foiled, and Stapleton is forced to return to Devonshire to once again pursue his evil objective there (Chapters 4 and 15).