Julien shoots Madame de Rênal because a letter from her to the Marquis de La Mole, which accused him of having tried to rise through hypocrisy and the seduction of a weak woman, undid the marriage with Mathilde and the future he had just won. The book, however, does not give the shot a simple motive.
The marquis, who had asked for information about Julien, receives the letter and writes to his daughter that he will never consent to the marriage. Julien reads both letters, says that the marquis "is just and prudent," leaves Mathilde behind, and sets out for Verrières. There he buys a pair of pistols and enters the church during Mass. At first he cannot: "I can't," he tells himself, "physically, I can't." He fires twice; the first bullet passes through her hat, and the second strikes her in the shoulder.
In prison he writes to Mathilde that he has taken his revenge. That same night, when the jailer tells him she has not died, he falls to his knees weeping, and only then does he begin to repent. Later, already condemned, he himself cannot say exactly what moved him. He asks how to convince Madame de Rênal that he loves her, after having wanted to kill her, and admits that he "wanted to kill her out of ambition or out of love for Mathilde."
The shot is the point where Julien's calculation fails for good: instead of maneuvering to save what is left, he destroys his own rise. And the letter, it is discovered in prison, had been written by the young priest who directed Madame de Rênal's conscience and had only been copied by her.