In that moment she pragmatically decided that, death or no death...well, why not?
So Sweeney Todd came to live in his former house turned pie-shop. Their twisted enterprise worked, steadily and against the natural order of things like a rusted cog in an old watch. Yet neither saw it for what it truly was. For her, well, it was just practicality, it was just - "Times is hard, Mr. T" - it was not being alone for once, it was - maybe - having the man she had dreamed of, even if it was nothing like what she'd dreamed.
For him it was a means to an end. With nothing but a pair of razors and a need for revenge like a need for air, he could right the injustices done to him and the world. He served people to people, and found it the most perfect of fates.
So to her deepest unhappiness, Mrs. Lovett found herself only his fellow conspirator, another leech on the murky underbelly of humanity--a business partner, if you will, to be mistreated and negotiated with. And if sometimes he saw her humming tunelessly as she baked their horiffic pies and thought, "That is a dark parody of someone I used to know", who had shared the same mundane and cheerful sort of movements, who had bustled in a kitchen full of light with flour in her yellow hair -- he never voiced it.
She was being used, and he was being tricked, and when all secrets were laid bare, she waltzed into eternal hellfire with him close behind.