Guilty conscience? It's a bit late for that...

Following his sister’s execution, all of a sudden Ferdinand feels bad (oh boo-hoo). In this passage, Ferdinand finally admits that his sister was married—“her marriage”—and, in addition, he states that it was his own fault—“That drew a stream of gall quite through my heart”—that his sister is dead. To me, it looks like he’s finally admitting that he was a greedy bastard, and also blind to the fact that his sister was wedded to Antonio. It's as though the realization, the actual seeing his sister dead, is what spurs his moral conscience to finally show up (a bit too late, I'm afraid).
This passage also mocks dramatic convention. Ferdinand states that actors are often “cursed” for “playing” a “villain’s part”—as though the actor is responsible for the character’s actions on the stage. For a moment, one might think he is talking about himself, but he is addressing Bosola. Ferdinand is aware enough of the fact that Bosola is merely an “actor,” and also that Ferdinand himself was the one who wanted her killed (“for my sake”), but he still can’t help but “hate” Bosola for what he did to his sister.