...an idea flashed through her mind, to fling herself into that death, as she would have done into any other, and to thrust Marius into it also. ...She died with the tragic joy of jealous hearts who drag the beloved being into their own death, and who say: "No one shall have him!"
In short, she leads Marius to the barricade after *withholding* a letter Cosette tried to send to him. Finding Cosette's house abandoned, he plunges into despair and is willing to die. Which indeed is an overreaction on his part, and Eponine does at least take a bullet for him and finally give him the letter, which I suppose evens out her final tally. But still. Not exactly cool, girl.
How come she gets all the good songs in the musical, dang it? Poor maligned Cosette.
In other news, I love that Victor Hugo is so precise about addresses, because it enables us to Google-Street-View them and peek at what's there today. Cosette and Valjean's house, containing the garden where Cosette and Marius meet in secret for a couple of idyllic months, is evidently at 55 Rue Plumet. Marius lives at 16 Rue de la Verrerie with his friend Courfeyrac. Those streets are both still there, not that they look much like they would have circa 1830. (I could find the Rue Plumet, but not a No. 55, and no gardens resembling Cosette's.) The barricade upon which they fight is in Rue de la Chanvrerie, and that confuses Google Maps, so the name probably got changed.