Character Analysis
If Ferdinand had a Facebook page, his Basic Info section would probably read something like this:
Sex: Male
Interested In: Sister, Radical Purity of Own Aristocratic Bloodline
Relationship Status: It's Complicated
Religious Views: I do think it is some sin in us heaven doth revenge by my sister
Political Views: Hopelessly corrupt; the law to me is like a foul black cobweb to a spider, I make it my dwelling, and a prison to entangle those shall feed me
Interests: Severed hands, parades of lunatics, werewolves, spies
Ferdinand is probably the weirdest character in this play (which is really saying something when you take into account that this play has Bosola), so bear with us.
What's Your Damage, Ferdinand?
The Duchess of Malfi has a complicated plot, and there are a lot factors getting thrown into the mix to make it that way. That said, arguably the strongest force moving things forward is Ferdinand's maniacal obsession with his sister, first focused on her chastity, and later on her destruction. From a distance, this makes the kind of sense that's… not: all three siblings are adults living separate lives, presumably in separate geographic locations. So why the heck is this guy so fixated on his sister?
If you're just going by what Ferdinand himself directly says on the matter, you won't get far. Bosola is the only person who comes close to asking him what his problem is with the Duchess remarrying, and Ferdinand just cuts him off, saying, "Do not you ask the reason, but be satisfied / I say I would not [have her remarry]" (1.1.250-51). Um, thanks for that illuminating comment, Ferdo.
Sure, Ferdinand mentions, at one point, that he hopes to inherit her wealth if she dies a widow (4.2.275), but the idea that he's just gunning for her property doesn't make a lot of sense. He mentions this inheritance thing once in the entire play, and doesn't have any plans on murdering her (her death would be the only way he would gain her property) until he finds out that she's defied his orders to stay single and chaste.
So it would seem that Ferdinand's obsession with the Duchess's chastity is a lot more complicated than the tried-and-true wife-murders-husband-for-the-insurance CSI plotline; it seems to stem out of the core of his deeply messed up personality.
Going To Extremes to Keep it in the Family
Think about how Ferdinand is introduced to us in the first scene of the play. He comes on stage, and Antonio says of him, "He speaks with others' tongues, and hears men's suits / With others' ears" (1.1.168-69).
In a weird way, Ferdinand lives through other people: he wants his courtiers to be his "touchwood," and to be extensions of himself; he "speaks with others' tongues." You can read these lines as meaning that there is no Ferdinand; that he's just other people, but it would probably be closer to the truth to say that, to Ferdinand, there are no other people.
See, he's trying to duplicate himself onto other people by inhabiting them—he doesn't want to be his courtiers, he wants them to be him.
Think about when he hires Bosola as spy: "It seems you would create me / one of your familiars" (1.1.252-53), Bosola says, and once he agrees to work for Ferdinand, he says, "I am your creature" (1.1.280). Ferdinand isn't just employing Bosola, he's recreating Bosola as an instrumental part of himself; as Ferdinand's "creature," Bosola becomes his proxy. Once you've wrapped your head around this twisted concept, Ferdinand's treatment of the Duchess starts to make a lot more sense.
Sister-Lover?
Ferdinand's behavior toward his sister has led a lot of people to conclude that he actually has incestuous feelings for her. Setting aside the ick-factor for a second, we can definitely see where they're coming from. Ferdinand's obsessed with the Duchess's sexuality and there are two instances where he threatens her with his Poniard of Phallic Symbolism (go check out the "Symbolism, Imagery, Allegory" section for more).
That said, in Ferdinand's case, so-called incestuous feelings may not translate to him wanting to actually have sex with the Duchess. Personally, we're taking the side of scholar Frank Whigham, who suggests that Ferdinand's "incestuousness" boils down to his obsession with his own purity.
Ferdinand is a proud, arrogant man, and a lot of that pride comes from his sense of superiority as an aristocrat. Think about the Duchess, who recognizes Antonio's virtues despite his low birth, and gives him new social identity as her husband. Ferdinand does the opposite, as Whigham notes: "he generates the loss of [his subjects'] identity, striving to become more himself by reducing others" (Whigham 169).
Nobles definitely conceived themselves as a separate breed of people at this time, but Ferdinand takes that idea and goes one step further: he narrows the idea of "his people" from "other aristocrats" to his direct family, and, even more specifically, to his twin sister. So ultimately, Ferdinand wants to control the Duchess so that he can control himself, as twisted as it sounds. If she's sexually active Ferdinand himself is violated. Ferdinand finally imprisons the Duchess to create a space where he can totally isolate her, and where only he (or his agents, like Bosola) has access to her.
And Then There's that Werewolf Part
How do you top a four-act run of incest, betrayal, and murder? Go totally crazy and become convinced you've turned into a werewolf, obviously. The onset of Ferdinand's lycanthropia comes soon after he's had the Duchess executed, such that it feels as though her murder has triggered it.
How did you interpret Ferdinand's madness? Some people figure that it's a sort of cosmic punishment for the terrible things he's done, but personally we're putting our money on it being a manifestation of Ferdinand's original character.
Ferdinand, as mentioned above, is not good at keeping his cool—when something upsets him he totally flies off the handle, and his more level-headed brother is frequently seen trying to get him to chill out, telling him, "[nothing] makes man so deformed, so beastly, / As doth intemperate anger" (2.5.57-58). Ferdinand's lycanthropia can easily be interpreted as his falling victim to the extremity of his own relentless, over-the-top fury; he's become the "deformed, beastly" thing his brother warned him about.
Ferdinand's transformation into crazy-wolf-man is the extreme conclusion not only of his anger, but also his fixation on his own peerless singularity. His doctor recounts how Ferdinand "Said he was a wolf, only the difference / Was, a wolf's skin was hairy on the outside, / His on the inside" (5.2.16-18). Ferdinand's obsessed with his own discrete purity, and his madness—the ultimate self-obsession; the ultimate solipsism—grants him what he's always wanted, in a way: he's finally totally alone and uniquely untouchable, bristling inside his own wolfish skin.