Hearing about the Dark Elves' prophecy, Lucien is left with conflicting feelings.
After reading his mother's letter, he thought she didn't want to influence or manipulate his decisions, but now he wonders if he really has a choice or if all his actions are being shaped by her.
Part of Lucien wants to be as rational and efficient as possible. Following the most evident path and allying with the Dark Elven Queen seems like the quickest option to end the war on the Shadow Star.
But what Lucien desires most deep in his heart is to do what gives him the most pleasure, which would mean ignoring all this mess and taking matters into his own hands.
As he contemplates this, a voice emerges within him. "Do it!"
That voice is clearly Lust's, who couldn't be happier with Lucien wanting to act on his desires above all, as it is what defines their sin.