Bosola and Ferdinand, undoubtedly the two most detestable characters in the play, play off each other, constantly shifting roles and changing the seeming balance of power. As the play goes on into the Acts 4 and 5, Ferdinand’s evil works further into the plot, while Bosola is portrayed less and less as the force of evil and more as the body through which Ferdinand is able to carry out his own evil wishes. Of course, Bosola is still a horrible person imo, but we really see Ferdinand come to the fore as he shows, even under pressure from Bosola, no remorse whatsoever for killing the Duchess. It is interesting to think broadly about how Bosola and Ferdinand’s respective roles change throughout the play, with Bosola’s agency appearing less in his own hands and more in the hands of Ferdinand as the acts play on. He is ultimately just the agent through which Ferdinand can carry out his psychopathic wishes, and while this does not excuse Bosola, it does present him less as an evil soul and more as a pathetic servant to Ferdinand, the true source of evil in the play.