Manipulation Perhaps?

I am not sure that Ferdinand is necessarily struck with morality all of a sudden. True, he finally recognizes the fact that he wanted his sister's wealth and the thought of her marrying (which would mean that he would lose all privilege) was what made him go insane, but that does not mean that he is repenting. Judging by his character throughout the play so far, it seems to me that, more than anything, he is using Bosola to relieve himself of any guilt involving his sister's execution. He is placing all blame on Bosola. I take the lines "a good actor many times is cursed / For playing a villain's part" as him telling Bosola that he as the actor (actor because he had no real reason to want the Duchess to die) was now cursed for committing a crime that was only in his (Ferdinand's -- the villain) interest. Therefore, Bosola was cursed for playing a part that was not meant for him, but for Ferdinand (I hope that made sense!).

The lines following the ones quoted in the coversation piece support this as Ferdinand goes on to guilt trip Bosola, literally telling him that he had no authority to do this because although he (Ferdinand) was angry and wanted her to suffer, he was not her "judge" which would have nulled any requests to have her killed. Essentially, I think Ferdinand is only spewing all of this to intimidate Bosola. He knows that if Bosola is fearing for his life/punishment, he will not open his mouth because even if he reveals that Ferdinand ordered or had any part in the Duchess's execution, he would also be charged as an accomplice. So basically, manipulating Bosola? Very likely. Regret? Not so much. I guess the next act will tell.