Both of the above quotations have elements of truth to them, but I think that to look at the character of Bosola, an audience would concentrate more on his actions and how they change his interpretation of the brothers, causing him to kill them at the end of the play. There are many layers to Bosolas character which encourage us to think that he is mentally unstable, e.g. killing the Duchess for his own self-interest as well as for the brothers. This does not make him a corrupt human being, but someone who has been forced into a corner, unsure of his actual status. Neither quote defines his change of self and portrays Bosola as a particular human being instead of one who transforms as a result of the corrupt society around him. He moves from our first impressions a murderer who is only out for himself...