Bosola is a complicated and fascinating character. He acts the role of both villain and avenger, working first against and then for the duchess.
First, Bosola agrees to work as a spy and also murder the Duchess for Ferdinand and the Cardinal, even though he knows they are evil and their court corrupt. He realizes he has entered into a wicked bargain. Ferdinand offers him a respectable job, something he covets very highly, as an ex-convict who spent seven years as a galley slave because of the murder he committed earlier. As Bosola puts it:
For the good deed you have done me, I must do
All the ill man can invent!
Later, however, the cynical Bosola grows to admire the Duchess for her genuine goodness. He sees the kind works of her "white hand." He wishes he worked for her, but he has made his bargain. He kills her, 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.