Start with Todd, which is where the story starts. He tells Anthony his story in song, about the "barber and his wife/And she was beautiful!/A foolish barber and his wife/She was his reason and his life!/And she was beautiful!/And she was virtuous!/And he was naive." The wife catches the eye of the true villain of the story, Judge Turpin; a man who has mistaken lust for love.
When Todd meets Mrs. Lovett, we learn with him more about the Judge's idea of love: after Todd was sent to Australia, Turpin lures her to his house where he rapes her during a costume ball. The party-goers, Mrs. Lovett (don't neglect that name!) tells Todd, thought she must be feeble-minded, and so watch the rape with sophisticated, not to say perverse and evil, delight. Before the play is over, everyone is villain, because everyone misunderstands the true nature of love.
Anthony, of course, sees Johanna, and it is for both of them love at first sight. Judge Turpin notices Anthony's attention and warns him away, but Judge Turpin announces to his beadle his plans to marry his ward, in order to "protect her."
There is more than a little selfishness in this play, which is perhaps the best opposite of love there is. Todd nurses his grief and injustices until he is a perfect monster of revenge. Even as he sings a love song to his razor blades, Mrs. Lovett sings a love song to him, one Todd is oblivious to, and there we see together two misdirected loves. Todd's only "friend" is his set of razor blades; Mrs. Lovett has apparently loved Todd from afar ever since he was the barber Benjamin Barker. Todd mourns Lucy, especially after Mrs. Lovett tells him about her rape and how she took arsenic afterwards, but as Todd has said to Anthony's question about Lucy at the beginning of the story: "All that was many years ago/I doubt that anyone would know." Many years ago, yet he nurses it still. His love for her has perverted into hate for Turpin which, when it seems he has lost his one chance to get Turpin into his barber's chair for vengeance, turns to hatred of all humankind. Todd's love is for Todd's hatred and lust for revenge.
And then there is the young boy, Toby, the assistant to Signor Pirelli who is himself the first victim of Todd's razor, just as Toby will make Todd the last. By the end of the story the boy sees Mrs. Lovett as his salvation, little thinking that she is as responsible for murder and horror as Todd is. He loves her as a child loves his mother, and while she is hardly a mother figure, she's the closest he's ever known. He tells her that: "Nothin's gonna harm you/Not while I'm around," and "Demons are prowlin' everywhere, nowadays/I'll send 'em howlin', I don't care, I got ways," the simple assurance of a child in a world of horrors he almost, but not quite, takes for granted. His is one of the closest to a clear moral vision the story supplies.
All of this misdirected love ends up as it must: Todd murders the beadle and the judge, but also murders his own wife, now a mad street person, and nearly murders his daughter. When he realizes the mad woman was his beloved Lucy, he gives Mrs. Lovett a fitting end, tossing her into the giant oven where she cooks her meat pies. Her young protector, hiding out in the cellar bakery and now himself quite mad, having seen the body parts and realized where the meat for the pies is coming from, slits Todd's throat as the barber grieves over the body of his dead wife.
There is no redemption here, no salvation, no Aristotelian recognition of responsibility, so it isn't even appropriate to call this grim tale a proper tragedy. In the play, when Toby reappears to slit the throat of a grieving Todd, his hair has turned white, the hoary Victorian chestnut that indicates a frightful shock.
Johanna's own ideas of love begin to change. Anthony goes to Fogg's Asylum to rescue Joanna, under pretense of being a wig maker's apprentice looking for blond hair of a certain shade. When Anthony confronts the asylum keeper with a gun, the director senses Anthony's reluctance to shoot, and Anthony drops the gun in despair. Joanna, however, takes it up, shoots, and they make their escape. She has lived the horrors of the asylum, if only for a few days, and she will do what must be done, what innocent Anthony cannot bring himself to do.
"The Ballad of Sweeney Todd" serves as the Greek chorus in the play; introducing the story and explaining the feelings of Todd as the story progresses. At the end, the ballad underlines the "moral" of the story:
And therein lies the dilemma, and the purpose behind the statement. It isn't what we love, or that we love; it is who we love, that most matters.
The play include the tragic elements of a chorus and a seeming acceptance of responsibility by Todd, in both the final scene and the opening and closing ballad. The ballad declares that Todd "serves a dark and a hungry god," with the implicit statement that he accepts the responsibility for such service. While Todd accepts responsibility for his crimes in the play, he is defiant to the end, slamming a heavy metal door on the audience on the final note of the ballad, ending the play in clangor.