It's not what you say but what you do...

It seems like Ferdinand is acting like a 10 year old boy in this passage! Imagine two boys getting bored and then one of them has an idea...like setting a neighbor's yard on fire...and then dares the other boy to do it; and then when the other boy actually does it, he gets mad and says, "why'd you listen to me?! you're an idiot!" He just won't accept the fact that the only reason why the Duchess is dead is because of him, not Bosola. He killed her long before Bosola strangled her; when the Duchess saw the "bodies" of her husband and children, that was the moment she died--there was no reason for her to continue living, her spirit was dead. I think that without having someone else to put the blame on, Ferdinand would go mad with guilt. Ferdinand confesses to the fact that it was "an infinite mass of treasure by her death" that inspired him to have her killed, but he also says, "And that was the main cause--her marriage, That drew a stream of gall quite through my heart." Perhaps it was the fact that his sister was sleeping with/married to another man that spurned an uncontrollable, jealous rage in him that ultimately led to her death.

Like the others below me have said, this passage is definitely another instance where the characters seem to be conscious of the fact that they are in a play. And I agree that when Ferdinand says "...a good actor many times is cursed for playing a villain's part," this line can be applicable to Ferdinand himself, and not just Bosola. Ferdinand may see himself as a "good" man/actor who is only acting as the villain. But we all know, it's not what you say but what you do that defines who you are. Ferdinand may not have strangled the Duchess, but he definitely killed her.