The blacklight reveals Saunière's last words and they, um, don't really clarify anything at all:
13-3-2-21-1-1-8-5 / O, Draconian devil! / Oh, lame saint!
Fache says they already have their cryptographers working on the numbers, and that the rest looks like some kind of accusation.
Also: Saunière was French. Living in Paris. Why'd he write his dying thoughts in English?
Then, Fache spread his beam over a broader area, and Langdon sees now that Saunière was reproducing The Vitruvian Man with his own body.
Fache thinks this is yet more proof that devil worship is involved, because Da Vinci was known to have a "tendency toward the darker arts". (Translated: He was gay, and a worshipper of Nature's divine order.) Hmm.
Fache has a point, though: Da Vinci did have a prankster nature, and he hid pagan symbolism in a bunch of his Vatican-commissioned Christian works.
Langdon proposes that maybe Saunière was expressing that he agreed with Da Vinci's spiritual ideologies.
After all, he also disagreed with the Church's elimination of the sacred feminine from modern religion.
Fache's pressing Langdon for answers, and Langdon is getting pretty exasperated because he doesn't really have any.
Fache thinks that Saunière was dying, and his intentions were to reveal his killer.
Langdon wonders why he'd write all the riddles and symbols. Saunière would've written the name of his killer—his mama didn't raise no fool, after all.
Fache smiles and says, "Precisely."
Back in Saunière's office, Lt. Collet's busy having a moment of hero-worship. See, what we didn't know is that Fache is convinced that Langdon is the one who killed Saunière, and this whole time he is just playing an intricate game to get Langdon to screw up and reveal his guilt. It's a process called cajoler, and Fache, a devout Catholic, is an expert at it.
Collet's job is to listen in on their conversation, and track Langdon using the digital tracker they'd put on him.