Here again the answer came rapidly and clearly.
"Because I didn't want him to destroy the will which I had tricked him into signing."
"How did you trick him?"
"He made me draw up a will which left all his property to Margaret Wallace."
"And then?"
"I drew up a will in which he left everything to me."
"And then?"
"I covered it with a sheet of paper, and got him to sign it, thinking that he was signing the other."
"Did he know what you had done?"
"Yes; I killed him before he could tell any one else and have the will destroyed."