This is a pretty good summary of the play, isn’t it?
Unfortunately, my student didn’t realize that: that their love led inevitably to their deaths (given their circumstances and personalities and impulsive youth).
Did she mean they loved each other a lot, as so many of us use this phrase: “Oh, I love you to DEATH, John!” (“To death” is used in this same way, to indicate a great amount or intensity, in such phrases as “he talked that subject to death!” and “she bores me to death.” Thus we can see that “to death” doesn’t necessarily carry a positive connotation–is, more often, negative…like death itself, I suppose…)
Judging from the rest of the essay she wrote, I have to think this was not her intention. No, it was an error much more likely: she had in mind the marriage vow to love the other “till death do us part.” Evidently what she had heard, or understood, when she attended weddings was “to death do us part.” So she was telling her reader that Romeo and Juliet kept that vow they probably murmured in Friar Lawrence’s monkish cell. They loved each other every minute until they died. They loved each other all the way to the moment of death. Her essay wasn’t even ultimately about the play; it was about love in general. Many people fall in love and get married and then they get divorced, unlike R&J, who were faithful to the end.
I will prefer to think that the sentence itself was a flash of insight, a conflation of Shakespeare’s entire play, and the fault was in the essay that didn’t live up to the moment of brilliance. A woefully brief visitation from Melpomene, the Muse of Tragedy.
And next time I go to see the play in question, as soon as the Chorus mentions the star-crossed lovers, I will think to myself, “Yes, they loved each other—they loved each other to death.” End of story!

She has already loved him to death; she’ll take his dagger and thus he will have also loved her to death. Death by loving. This engraving, an illustration, from an old book (note “Tales from Shakespeare” in the margin), is all over the Internet. No wonder.