He revolved the means which might effect his purpose--he saw but
one--this was the death of the marchioness.

The commission of one crime often requires the perpetration of
another. When once we enter on the ladyrinth of vice, we can seldom
return, but are led on, through correspondent mazes, to destruction.
To obviate the effect of his first crime, it was now necessary the
marquis should commit a second, and conceal the imprisonment of the
marchioness by her murder. Himself the only living witness of her
existence, when she was removed, the allegations of the Padre Abate
would by this means be unsupported by any proof, and he might then
boldly appeal to the pope for the restoration of his child.

He mused upon this scheme, and the more he accustomed his mind to
contemplate it, the less scrupulous he became. The crime from which he
would formerly have shrunk, he now surveyed with a steady eye. The
fury of his passions, unaccustomed to resistance, uniting with the
force of what ambition termed necessity--urged him to the deed, and he
determined upon the murder of his wife. The means of effecting his
purpose were easy and various; but as he was not yet so entirely
hardened as to be able to view her dying pangs, and embrue his own
hands in her blood, he chose to dispatch her by means of poison, which
he resolved to mingle in her food.