For I do play a part in't 'gainst my will. (4.1.81-82)
Meta-alert. Duchess of Malfi is known for bringing attention to its own theatricality—it's full of theatrical device and frequently features instances of the characters themselves being turned into an audience watching other characters perform.
I am not mad yet, to my cause of sorrow.
Th'heaven o'er my head seems made of molten brass,
The earth of flaming sulphur, yet I am not mad;
I am acquainted with sad misery
As the tanned galley-slave is with his oar. (4.2.23-28)
"Gee, I wish I were crazy" isn't something you hear much, but if you were being tormented the way the Duchess is by Ferdinand you'd probably wish you were nuts, too. Not only is earth a terrible place, but even heaven is figured as a kind of inferno—she's actually quoting a bit of the Bible talking about God's smiting methods. This is also the point at which Bosola starts undergoing a change of heart, partly because he realizes the Duchess shares his attitude about the world. The Duchess here likens herself to a galley-slave, which is exactly what Bosola was for many years as punishment for his work for the Cardinal.
And Fortune seems only to have her eye-sight
To behold my tragedy. (4.2.35-36)
We don't blame the Duchess for being such a Debbie Downer. After all, she's being tortured by Ferdinand, and is going to be executed in the not-to-distant future. Unlike Bosola, who ends up thinking that Fortune, if it exists, is basically random, the Duchess thinks that the deck is actually stacked against her, and, truth be told, she's kinda right.
[…] Didst thou ever see a lark
in a cage? Such is the soul in the body: this world is like her
little turf of grass, and the heaven o'er our heads like her
looking-glass, only gives us a miserable knowledge of the
small compass of our prison. (4.2.122-26)
Here's part of Bosola's closing performance to bring down the Duchess. Even though the Duchess is literally Ferdinand's prisoner, Bosola tells her that her real prison is her earthly body. We're all trapped, knowing just enough about the world to be aware of our entrapment. Ferdinand is trying to drive the Duchess crazy—to take her out of her body—but Bosola's trying to show her that the real torture is being confined inside of it.
Of what is't fools make such vain keeping?
Sin their conception, their birth weeping;
Their life, a general mist of error,
Their death, a hideous storm of terror. (4.2.181-84)
By now, you should hear these words and think, "hey, that sounds very Bosola-ish." Why? Because it describes a world that isn't just bad (there are several characters who would be onboard with "sin their conception, their birth weeping"), but is, moreover, randomly bad. Bosola's idea of life as a "general mist of error" is validated by his own experiences. How so? Well, more than any other character, Bosola has a bird's eye view of society's corruption, but despite that knowledge even he can't game the system. When he tries to fight back against that "mist," and save Antonio, he ends up accidentally killing him.
The Cardinal: There is a fortune attends thee.
Bosola: Shall I go sue to Fortune any longer?
'Tis the fool's pilgrimage. (5.2.294-96)
The Cardinal is commissioning Bosola to murder Antonio, promising him a whole lotta dough—a "fortune," to be exact. Bosola, in one of many instances of bleak double meanings of this play, turns "fortune" back on itself. He mocks not only the idea of fortune as reward (he's seen with Ferdinand how far the promise of reward gets him), but also the larger concept of Fortune, or Fate. To Bosola, there doesn't seem to be any coherent guiding force in the universe, and if there is, it's hardly sympathetic to the suits of mortals.
Thou in our miseries Fortune have a part,
Yet in our noble suff'rings she hath none.
Contempt of pain—that we call our own. (5.3.54-56)
Here's Antonio, speaking to Delio right before he sneaks into the Cardinals' palace to try and talk things out with him. His attitude is distinctly different from Bosola's. Yeah, Fortune may have something to do with the fact that we suffer in the first place, but the dignity with which we handle that suffering is all down to our exceptional human-ness.
Think about this quote alongside the Duchess's experiences during her imprisonment and execution: many would argue that her comportment during those scenes exemplifies exactly what Antonio's talking about here. She suffers, but does so nobly, and her "contempt of pain" (and, ultimately, the world at large) is frequently cited as a iconic example of human dignity.
We are merely the stars' tennis balls, struck and banded
Which way please them. (5.4.54-55)
There's been plenty of talk about Fate having it in for our characters, but here Bosola eighty-sixes the entire idea. If Fate exists (here characterized as the "stars") it's not some solemn guiding force that's deliberating over your existence, it's just deciding at random; The Powers That Be are basically playing ping-pong with your life. Bosola says this right after he accidentally kills Antonio, realizing that he lives in the same "general mist of error" (see Quote #5) as everybody else.
Pleasure of life, what is't? Only the good hours
Of an ague; merely a preparative to rest,
To endure vexation. (5.4. 67-69)
Check out some of Antonio's last words, uttered right after he's been stabbed by Bosola and then, finally, told that his wife and kids are dead. Despite everything that's happened, Antonio's been a pretty relentlessly glass-half-full kind of guy up until now. While he's previously voiced his belief that "Heaven hath a hand" in their sufferings (3.5.61), he's never, until now, embraced the Duchess's own wholesale-despair-of-life-itself outlook. Given that he just found out that his family, which he previously thought was safe, has been murdered, and that he himself is going to be joining them in about five seconds, we don't blame him.
Oh, this gloomy world!
In what a shadow, or deep pit of darkness,
Doth womanish and fearful mankind live! (5.5.99-101)
These are Bosola's final words, spoken in his characteristically cheerful fashion. Bosola's idea that (a) the world sucks and (b) it sucks in really random, unpredictable ways is validated by his own death. He succeeds in killing the bad guys, but not only is he himself not rewarded for championing good (he himself is killed), he isn't really punished for it either. Bosola's death comes about more or less by accident in the final desperate, scrabbling murder scene, and the staging of it doesn't make it look like "The good guy gets smacked down by Fate," it just looks like he dies in the murderous, messy version of a fender-bender. The "deep pit of darkness" is so obscure that can't be any directing force, either on the part of man or heaven or Fate.
These wretched eminent things
Leave no more fame behind 'em than should one
Fall in a frost and leave his print in snow:
As soon as the sun shines, it ever melts,
Both form, and matter. (5.5.112-16)
Key question here: are you buying this? Go check our "What's Up With the Ending?" section for some more thoughts on this, but Delio's closing statement that all of the bad stuff that's been going down will just melt away and the sun will come out is kind of dubious. By which we mean, really, really super dubious.