Dostoevsky provided his protagonist Raskolnikov with a number of doubles and foils: Marmeladov is a sort of double, a “criminal” with the best of intentions; Razumikhin a foil, strong, generous and well-balanced where Rodya is self-absorbed erratic—but equally passionate and warm-hearted. Even the hypocritical, self-important Luzhin provides an ironic contrast to Raskolnikov when Luzhin mouths the radical theories that have attracted Rodya and shows how empty they are.
Svidrigailov is another sort of “double,” who like Rodya is haunted by his conscience through dreams and ghosts. “He’s mad,” is what Rodya correctly thinks of his mirror image—and of course Rodya is mad, too.
Look for the ways Svidrigailov provides a counterpoint to Raskolnikov in his actions, his rationalizations, his self-absorption, his egotism, his crimes, and his agony. To help you make the connection, I’m quoting Joseph Frank’s introduction of this serpent-like character below, in which he in his turn quotes the English Romantic poet Lord Byron:
One of Dostoevsky’s most strangely appealing characters, a sort of monster à la Quasimodo longing for redemption to normalcy, Svidrigailov is much less a melodramatic villain…His Byronic world-weariness signifies a certain spiritual depth, and the contradictions of his personality, which swing between the blackest evil and the most benevolent good, perhaps can best be understood in Byronic terms. Is he not similar to such a figure as Byron’s Lara, in the poem of the same name, “who at last confounded good and ill,” and whose supreme indifference to their distinction made him equally capable of both? One can well say of Svidrigailov:
Too high for common selfishness, he could
At times resign his own for other’s good,
But not in pity, not because he ought,
But in some strange perversity of thought,
That sway’d him onward with a secret pride
To do what few or more would do beside;
And thus some impulse would, in tempting time,
Mislead his spirit equally to crime.Svidrigailov thus embodies the same mixture of moral-psychic opposites as Raskolnikov, but arranged in a different order of dominance. What rules within him is the conscious acceptance of an unrestrained egoism acting solely in the pursuit of personal and sensual pleasure; but his enjoyments are tarnished by self-disgust. What dominates in Raskolnikov are the pangs and power of conscience even in the midst of a fiercely egoistic struggle to maintain his freedom. Svidrigailov also resembles Raskolnikov in the sophistication and sharpness of his intellect; he is a brilliant and witty talker who does a great deal to enliven the final sections of the book.
(from pages 129-130 of Dostoevsky: The Miraculous Years, 1865-1871 by Joseph Frank. Princeton University Press, 1996.)