Death and diseases through the whole land spread. (1.1.5-15)
Antonio's describing the French court, which he just visited, to his buddy Delio. This is the first of many "models" of good and bad government you see throughout the play, but this one is particularly significant because it gives the audience a blueprint of what government can and should be, and one which is sharply contrasted against the corrupt Italian courts. Additionally, it introduces the all-important prince-to-courtier dynamic, and imputes the health or disease of the social system to the integrity of the prince, the fountainhead of the system.
Methinks you that are courtiers should
be my touchwood, take fire when I give fire, that is, laugh
when I laugh, were the subject never so witty. (1.1.120-22)
This is Ferdinand, who would probably call the French king a sucker if he met him. In contrast to King Frenchie, Ferdinand is (a) all about the "flatt'ring sycophants" and (b) a much bigger fan of the idea of the courtiers just copying him—none of this mutual symbiotic nonsense the French are into. As opposed to having the courtiers and ruler sustain each other, Ferdinand just wants to duplicate himself among his sphere of influence.
He and his brother are like plum trees that grow crooked
over standing pools: they are rich, and o'erladen with fruit,
but none but crows pies and caterpillars feed on them. (1.1.48-50)
Another form of government, this time illustrated by Bosola, "the only court gall," who has few illusions about the Cardinal and Ferdinand. He's describing natural objects acting in a very unnatural way. Instead of nourishing the ground and water underneath them with their bounty, the "trees" of the brothers hoard their wealth and thereby bring a total halt to the natural process of generation.
A good question, though, is whether you think that Webster indicates elsewhere that fair, just rule is the natural way of the world, or if the real world favors the corruption of the Cardinal and Ferdinand. You gotta wonder, though: if Bosola thinks these guys are so bad, why does he work for them?
[…] places in the court are but like beds in the hospital,
where this man's head lies at that man's foot, and so lower,
and lower. (1.1.65-67)
Bosola is once again (surprise, surprise!) complaining about the court. Here, he's deploring the vicious cycle courtiers get stuck in as they scrabble over each others' backs for the prince's favor. Bosola describes the system of courtly advancement in terms that carry the same tone as his crooked trees metaphor from Quote #2, in that he's presenting us with a picture of social rot, instead of generation or health.
Where he is jealous of any man he lays worse plots
for them than ever was imposed on Hercules, for he strews
in his way flatterers, panders, intelligencers, atheists, and a
thousand such political monsters. (1.1.155-58)
Antonio is painting a not-too-flattering portrait of the Cardinal for his friends Delio. He's a terrible guy in general, but in particular he's the Terrible Guy version of a politician—he surrounds himself with "political monsters," a bunch of useful but totally disposable people the Cardinal can use to boost his own power. In the Cardinal, you see the decay of Christian values in all of its ugly glory. He has absolutely no sense of ethics, and his position within the church is simply another platform from which he can amass power.
For know: an honest statesman to a prince
Is like a cedar planted by a spring;
The spring bathes the tree's root, the grateful tree
Rewards it with his shadow. You have not done so.
I would sooner swim to the Bermoothes
Than depend on so changeable a prince's favour! (3.2.258-65)
Bosola is criticizing the Duchess for axing Antonio, telling her how a court should work. Contrast this with the tree-pool metaphor he uses in Quote 3 to describe the perverse rule of the Cardinal and Ferdinand. In these later lines, Bosola rewrites the tree-spool image to describe something much closer to the French king's system, whereby ruler and ruled are nourished by each other and flourish.
These factions amongst great men, they are like
Foxes: when their heads are divided
They carry fire in their tails, and all the country
About them goes to wrack for it. (3.3.36-39)
These lines are spoken by Pescara, a minor noble who's definitely wise to the dark side of the courtly scene. Instead of acting as a unified, guiding body, "great men" are always fighting amongst themselves and tearing apart the countries they're meant to be leading.
Much you had of land and rent,
Your length in clay's now competent;
A long war disturbed your mind,
Here your perfect peace is signed. (4.2.172-174)
Up until the very end, the Duchess insists upon her title as prince, and in final stages of Ferdinand's plot to "bring her to mortification" Bosola's pointing out that, in the grand scheme of things, rulership means diddly squat. All of this power that the Duchess claims to have, he says, is basically imaginary—the only real land she owns is the clay that makes her coffin.
I am Duchess of Malfi still (4.2.134)
Easily one of the most famous lines of this play, although people definitely disagree on how they feel about it. The Duchess is in her darkest hour, imprisoned by Ferdinand, and about to be executed. In the face of Bosola's attempts to freak her out as much as possible and her imminent death, the Duchess's utterance of these words is interpreted by plenty of readers as an impressive, dignified response to a horrifying situation. But, you can also see this is a sad moment where the Duchess is clinging to a title that imprisons her more than it gives her power. She's investing in the very system that's destroying her.
[…] in all our quest of greatness,
like wanton boys whose pastime is their care,
we follow after bubbles blown in the air. (5.4.64-66)
Why the hell are we doing any of this? Thanks, Bosola, we were kind of wondering ourselves. Princes and courtiers alike are caught in the vicious cycle described by Bosola in Quotes 3 and 4, but what's the payoff? "Greatness" is fleeting and, even in the moment, immaterial—this image is particularly interesting because it figures power, and by extension the terrible social apparatus that supports it, as something that's basically imaginary.