Before the opera begins, Donna Elvira has been seduced by Don Giovanni who has promised to marry her. She spends much of the opera looking for him, finding him, and thwarting his new "conquests," all the while crying out for revenge and simultaneously trying to get him back. She is, it would seem, more than a tad conflicted.
Knife in hand, she sings a soliloquy in which she threatens to rip out Don Giovanni's heart if she finds him--not knowing that the very knave who betrayed her is listening. He, on the other hand, doesn't recognize her voice--which speaks volumes about how much attention he's paid to her--and at the end of her "aria" decides to step out and "comfort" her, as only the Don can do. Of course, there's the question of who will recognize whom first--and what the result will be!
Elvira: Ah who will ever tell me where that barbarous man is, whom I shamed myself for and loved, and who broke the faith with me? If I find that wicked man and he doesn't come back to me, I'll ruin him: I'll tear out his heart!