They had already arranged their infamous plot ”that he should marry this rich, but rather foolish old lady, induce her to make a will leaving her money to him, and then gain their ends by a very cleverly conceived crime. 
They became rich thanks to their murder plot.
mnli hypothesis: They became rich thanks to their murder plot. premise: They had already arranged their infamous plot ”that he should marry this rich, but rather foolish old lady, induce her to make a will leaving her money to him, and then gain their ends by a very cleverly conceived crime. 