as is expected. However, when Ferdinand reneges on paying Bosola, Bosola gets angry. He says,

I served your tyranny, and rather strove
To satisfy yourself than all the world;
And, though I loathed the evil, yet I loved
You that did counsel it, and rather sought
To appear a true servant than an honest man . . .

Killing the Duchess, Bosola says, was "much 'gainst mine own good nature." He decides to avenge the Duchess by killing her brothers, though he is killed in return.

In The Duchess of Malfi,  the audience has no misconceptions about Bosola, the malcontent, released from prison despite his part in a vicious murder. Bosola is not happy with his status in life and is determined to better himself at all cost, failing to recognize his own contribution to his misery to the point that, as Antonio points out, he will "poison all his goodness."  The totally contrasting and conflicted nature of Bosola, his ability to convince the Duchess that he can be trusted and his belief that he is just "the bellman" in all the schemes and murders, confirms his shallow nature and his insular personality.  

Bosola is aware of the duplicity of the society to which he belongs and uses it to his advantage. Initially impressed by the Duchess as she sees value in a person regardless of their social standing and "merely for worth...," he still spies on her on behalf of her brothers. His character supports the theme of appearances as compared to reality and, although his character develops and his perception changes, it is not sufficient for him to actually rise above his wickedness in order to prevent further tragedy.There are a few acts of kindness that Bosola seemingly, carries out but ultimately, he sees no purpose in good deeds as they always backfire, on him, particularly.  He realizes that the Duchess and Antonio have real meaning in their relationship and that being a good servant by spying on the Duchess has done anything but make him a good man. In order to redeem himself, he intends to kill Ferdinand but mistakenly, kills Antonio. Just before his own death, he reflects on the "deep pit of darkness," in which everyone must exist. It is small consolation to him that Ferdinand and the Cardinal are dead.

In John Webster's The Duchess of Malfi, Bosola is a somewhat ambiguous character who has no problem committing great evil with an extreme ruthlessness yet is also strangely drawn toward good by the end of the play. Let's look at this in more detail.

Bosola is a spy for the Duchess' brothers, Duke Ferdinand and the Cardinal, who want to prevent their sister's marriage and thereby gain control of her estates. Bosola does his job well, fitting into the Duchess' household as the master of horse and discovering that the Duchess has secretly had a child. No one knows that the Duchess is actually married to her steward, Antonio. Even Bosola has not discovered this.

Unfortunately, the Duchess trusts Bosola too much and engages him to get Antonio out of danger. Bosola at once betrays the Duchess and Antonio to the Duke and the Cardinal. He then captures the Duchess and takes her to the Duke as a prisoner. Eventually, acting on her brothers' order, Bosola commits his greatest evil and strangles the Duchess to death, also killing her maid and two of her children.

But Bosola soon realizes that the Duke and the Cardinal are not at all trustworthy. He does not receive his expected reward, and he drops his allegiance to the brothers. He feels some guilt at his murder of the Duchess (as well as a strong resentment that he has not been given his expected reward), and he decides to join with Antonio to avenge the Duchess. In the end, Bosola kills both the Duke and the Cardinal but accidentally kills Antonio in the process.

It seems, then, that Bosola is not totally evil. He is not successful in achieving good by the end of the play. He is still a murderer several times over, but he has at least recognized the evil of his act in killing the Duchess, and this is a start. Unfortunately, Bosola dies before he can find much in the way of real good.

Bosola is a hired killer and spy, a man who did prison time for murder before being hired as an "intelligencer" (spy) by Ferdinand to infiltrate the duchess's household. What tempts him to take the job is that he, a criminal drifter, will have the high-status title he craves: he will be the duchess's "Gentleman of the Horse."

Bosola is notable for his clear-eyed sense of reality. Unlike his employer, Ferdinand, who can't keep a grip on reality, Bosola has had to live by his wits. Therefore, he always knows the score. For Bosola, what he calls "gratitude," or what we might call a transactional approach to life, is the moral code he lives by. He accepts that there is always a price: he knows he will never get something for nothing, and he will never do something for nothing. He expects to pay, and he expects payment. He says when he accepts the position of Gentleman of the Horse in return for spying,

Oh that to avoid ingratitude
For the good deed you have done me, I must do
All the ill man can invent!

In other words, he would love to be Gentleman of the Horse without having to do "all the ill" (dirty work) Ferdinand has in store for him. He wishes he could go against Ferdinand, take the title (the good part), and forget about the gratitude or debt he owes his employer. But Bosola would never behave that way.

"Gratitude," or the transactional code, is all-important to this tool of Ferdinand's evil deeds. When Ferdinand turns on him and refuses to reward him (is ungrateful) after Bosola does the ultimate dirty work of killing the duchess and her children—the horror of which touches even Bosola's hard heart—this so offends Bosola that he switches sides and seeks his own revenge on Ferdinand.

Bosola is realistic and hardheaded. He sees corruption as only a servant figure who actually does the dirty work can, and he can act with efficient ruthlessness. At the same time, he has a code of honor, and when it is violated, he shows that he won't be toyed with.