Do you think Henri is insane by the end of the book? How so?

This is open to speculation. After falling for, and ultimately being rejected by, Villanelle, Henri retreats into his own mind, creating a world part real, part memory. In this reverie Henri is able to divest himself of hope, and instead live in a now of his own choosing. Though perhaps mad, Henri is conscious of his condition, seeing it as a product of free will. In a universe of one, Henri at last cannot be betrayed.