A hopeful note to start the play on, no? Bosola serves up a thesis statement that the rest of the play investigates: you can be good, but nobody's going to reward you for it. There's another side to these lines, though. The Renaissance was an time when the old feudal system of allegiance and favor was passing over into a system of employment and service, and when Bosola speaks of "virtue," he's figuring it not so much as a personal quality as a commodity, a form of exchangeable labor for which he feels he should be rewarded.
I would have you curse yourself now, that your bounty,
Which makes men truly noble, e'er should make
Me a villain: oh, that to avoid ingratitude
For the good deed you have done me, I must do
All the ill man can invent. (1.1.264-68)
Whew. So much going on here. Bosola is playing on the dual meanings of the words "noble" (both as "an aristocrat" and "virtuous") and "villain" (both as "a low-born person" and "criminal") to describe his dilemma with Ferdinand. This is the moment when Ferdinand truly gets Bosola on board to spy for him, and notice that it all hinges on this word "gratitude." Bosola's definitely happy for the actual position that Ferdinand's wrangled for him (provisor of the horse), but moreover he's indebted to him because Ferdinand has done him a favor. "Gratitude," then, isn't really being used here as the wibbly-wobbly emotional concept we have today, but rather as a hard-and-fast social dynamic.
Can this ambitious age
Have so much goodness in't as to prefer
A man merely for worth, without these shadows
Of wealth and painted honours?
that some preferment in the world can yet
arise from merit. (3.2.272-82)
Bosola's just found out that the Duchess and Antonio are married, and he's celebrating the fact that she chose Antonio despite the fact that he's her social inferior. There's a lot of debate over whether or not this speech is genuine, or if Bosola's just trying to get on the Duchess's good side. See, even though he's a spy, Bosola is consistently very straightforward with his moral judgments. And although he's obliged to go tell the Duchess's brothers what he's learned, it's totally reasonable to believe that his joy that "some preferment can yet arise from merit" is authentic.
Why didst not thou pity her? What an excellent
Honest man might'st thou have been
If thou hadst borne her to some sanctuary
Or bold in a good cause, opposed thyself
With thy advanced sword above thy head
Between her innocence and my revenge! (4.2.263-68)
After he's carried out Ferdinand's orders to murder the Duchess, Bosola finds out that Ferdinand has no intention of rewarding him. Ferdinand tells Bosola that he's the real bad guy for following Ferdinand's orders, and should have protected the Duchesses from him. This basically the total reversal of the "gratitude" that Bosola thinks of as binding him to Ferdinand in the first place; he's not going to reward Bosola for his service, and in fact refutes the very terms of their relationship.
Your brother and yourself are worthy men,
You have a pair of hearts are hollow graves,
Rotten, and rotting others; and your vengeance,
Like to chained bullets, still goes arm in arm.
You may be brothers: for treason, like the plague,
Doth take much in a blood. I stand like one
That long hath ta'en a sweet and golden dream:
I am angry with myself now that I wake. (4.2.308-14)
Here's the moment where Bosola shakes off the chains of service to Ferdinand. Bosola voicing his less-than-flattering opinion of the Aragonian brothers is nothing new, but it's interesting to hear him characterize his service to them as "a sweet and golden dream." He's hated these guys all along, but his hopes for a rewarding relationship with them has kept him "asleep," and allowed him to ignore his own morality.
[…] whilst a guilty conscience
is a black register wherein is writ
all our good deeds and bad, a perspective
that shows us hell […] (4.2. 346-49)
The Duchess has revived for a moment only to, darn it, die again, this time permanently. Bosola, now regretting his actions, describes his own conscience (which, Breaking News, he does indeed have) as a record of all of his actions, which also acts as a mirror that shows hell. It's worth mentioning that this is a really determined view of ethics. You do a thing, it's either a good or a bad thing, and depending on which you will see hell or something else. How does this compare with the way Bosola feels about morality by the very end of the play?
There are a many ways that conduct to seeming
Honour, and some of them very dirty ones. (5.2.298-99)
The Cardinal has just asked Bosola to murder Antonio, promising that there are "honours" in store for Bosola if he goes through with it. There are, of course, multiple meanings for "honor" at work here. You have the moral concept of honor—conducting yourself in an honorable manner—and then there's what the Cardinal is implicitly offering Bosola, which is social distinction and reward. Bosola here is remarking upon the irony that, if you want the kind of honor that matters in courtly society, you have to do some pretty dishonorable things.
O penitence, let me truly taste thy cup,
That throws men down, only to raise them up. (5.2.339-40)
Raised up sounds pretty good, right? Bosola, having just decided to avenge the Duchess, sure thinks so. Having been bitterly disillusioned about his relationship with Ferdinand, Bosola now seeks the rewards of righteous revenge, thinking that he can depend upon Penitence (rather than a corrupt nobleman) to pay him back for his good work. And, well, we know how this turn out. The Duchess of Malfi: Where Good Guys Don't Just Finish Last, They Finish Dead.
How tedious is a guilty conscience! (5.5.4)
These are definitely not the words that you were expecting from the Cardinal. He's pacing around his palace, presumably thinking about the freshly murdered Julia and his slightly-less-than-freshly murdered sister. Do you think he's sincere? If you were the actor playing the Cardinal, would you make this sound sarcastic, or would you want it to sound as if the Cardinal is actually troubled by what he's done?
My sister! Oh my sister, there's the cause on't!
"Whether we fall by ambition, blood, or lust,
Like diamonds we are cut with our own dust." (5.5.70-72)
Ferdinand's final words are, perhaps not surprisingly, about his sister. When he first learned that the Duchess had given birth, he said that it was as if "some sin in us heaven doth revenge / By her" (go look at Quote #2 in Family for the full picture). While the latter part of these lines can easily translate to "we're all brought down by our own deeds," what do you make of the fact that Ferdinand's "own dust" is, to him, equivalent to "my sister"? Do you think that "the cause" that he mentions refers to what the Duchess herself did, or what Ferdinand did to her?