In what way did Stapleton made use of the fact that Sir Charles had a weak heart in Hound of the Baskervilles?
Stapleton made use of the fact that Sir Charles had a weak heart to devise a way to kill him without being detected as the murderer.
Sir Charles was one of only two people who stood in the way of Stapleton inheriting the valuable Baskerville estate. Using to his advantage the fact that no one knew of his own identity as a potential heir, Stapleton moved into the neighborhood of his ancestral home and set about befriending Sir Charles and finding out as much as he could about the old gentleman. From Dr. Mortimer, Sir Charles' friend and personal physician, Stapleton learned about his relative's heart condition, and also found out that he took the grim legend about the spectral hound which haunted the Baskerville family very seriously. With this information, Stapleton's "ingenious mind instantly suggested a way by which the baronet could be done to death, and yet it would be hardly possible to bring home the guilt to the real murderer".
By securing a great hound and camouflaging it with phosphorous so that it would look like it breathed fire, Stapleton was able to waylay Sir Charles one evening and literally scare him to death. The sight of the horrid apparition coming after him off the desolate moor caused Sir Charles to run in extreme panic for his life, and the combination of exertion and terror proved to be too much for his weak heart. With the death of Sir Charles, Stapleton was one step closer to inheriting the fortune he so coveted (Chapter 15).