She will give him up to the Queen, for the sake of his career; and she tells the Queen, before Norbert, that he has confessed to her his love for the Queen--another lie! Norbert is indignant--he may well be--and throws down all this edifice of falsehood.

The Queen knows then the truth, and leaves them in a fury.

Constance and Norbert fly into each other's arms, and the tramp of the soldiers who come to arrest them is heard as the curtain falls.
I do not believe that Browning meant to make self-sacrifice the root of Constance's doings.

If he did, he has made a terrible mess of the whole thing.

He was much too clear-headed a moralist to link self-sacrifice to systematic lying.