Why did Stapleton use the legend of the hound to kill Charles?

Stapleton was actually the nephew of Sir Charles. His real name was Rodger Baskerville, and he was determined to gain his inheritance.

“Yes, it is an interesting instance of a throwback, which appears to be both physical and spiritual. A study of family portraits is enough to convert a man to the doctrine of reincarnation. The fellow is a Baskerville—that is evident.”
“With designs upon the succession.”
“Exactly. This chance of the picture has supplied us with one of our most obvious missing links. We have him, Watson, we have him, and I dare swear that before tomorrow night he will be fluttering in our net as helpless as one of his own butterflies. A pin, a cork, and a card, and we add him to the Baker Street collection!” He burst into one of his rare fits of laughter as he turned away from the picture. I have not heard him laugh often, and it has always boded ill to somebody.