Although the escape of the lovers does create a sort of happy ending for the poem, the final stanza does include a tragic tone.
Tone is defined as the author’s attitude toward the subject. To determine tone, it is best to examine the connotations of words that an author chooses.
From the final stanza of Keats's “The Eve of St. Agnes,” the following list of words have an extremely negative connotation: "gone," "storm," "woe," "shade," "witch," "demon," "coffin," "be-nightmar’d," "palsy," "ashes," and "cold."
The first two words in this list come from the last line, in which the runaway lovers are mentioned. Rather than saying they left together, the word “gone” suggests a sudden disappearance. In addition, they left during an “elfin storm,” which could symbolize the risk and danger of their decision. This is not exactly a happy ending.
Overwhelmingly, the words from this stanza are associated with death. Knowing that the Eve of St. Agnes is a holiday that relates to portentous dreams, the reader can infer that the party guests at the Baron’s castle have ominous dreams of death and destruction. This is in sharp contrast to the supposed happy resolution of the two young lovers’ story. Madeline’s parents, while asleep in their beds, actually die at the end of the poem. One could argue that Madeline’s choice to abandon her parents and run away with Porphyro breaks their hearts. Because they are not awake when she makes that choice, they simply die in their sleep. This explains the portentous dreams of the other guests: they predicted Angela and the Beadsman’s fate.