Don't you just love it when they wink at the camera?

I think that Ferdinand's lines here speak (as discussed below) simply another chance for Webster to point out to his audience that they are watching a play. I think that this one is especially important, since the actor here is basically telling the audience to not blame him for all of the terrible things that he's had to do in service of this play. I think that the larger point is the fact that he's really pulling out that Shakespeare line (was it As You Like It?) about the whole world being a stage... while there's obviously wordplay going on in a meta-theatrical sense, on the diagetic level the line still manages to resonate. Ferdinand has done these terrible things, he's hired Bosola, and now he speaks of the things that drove him to assume the character of the villian. He's making excuses for all of this terrible deeds in a very roundabout way, by accusing Bosola directly. I think that he's slyly referring to himself, in the sense of being "a good actor many times cursed."