Rael Aveross was a former apprentice to Count Dooku, Dooku's last apprentice before he took on Qui-Gon. Rael had a close relationship with Dooku and was keeping in contact with him up until atleast the end of Master and Apprentice. Rael wasn't known as a "by the book" Jedi, and he had a backstory laced in tragedy--inadverdently causing his own apprentice's death, he came to care for a young Princess he was ordered to protect, guard, teach and practically raise until she was ready to govern. He was isolated from the Order, this being his one duty, and he got caught up in some... let's say, extracurricular activities. He was blinded by his love and devotion for the young girl to where he allowed himself to be played for a fool by her plotting behind his back. His relationship with Qui-Gon is one of brothership, him being the older one, and they endured some conflict with one another before eventually returning to a place of mutual respect and admiration. But even so, while he ends the story on a narrative "high note," declining Dooku's offer to join him in the dark, he is still very much not a completely level-headed and dare I say rational individual, someone I think could be spurned onto the kind of extremism we see Luthen partake in should he have experienced multiple tragic events on-top of what he had already been through before and during his debut novel.
If we are to assume Luthen Rael is indeed Rael Aveross, then the list of tragedies he would have undergone is the following:
-Causing the death of his apprentice
-Allowing himself to be blinded by his attachment to the young Princess, having her play him for a fool, almost costing many their lives.
-Losing his master to the dark side
-Losing his younger brother figure to death, to the sith
-Seeing the order he served and, in many ways, became disillusioned with, fall further with the rise of the clone wars
Perhaps Luthen's investment in the Rebellion could be tied to the lack of action he took during the last years of the republic, thus him feeling guilty of not doing enough then. But he's presented with an opportunity now to right that wrong.
It's the greatest piece of dialogue we've had in all of Star Wars screen media, at the very least
Not as iconic as the most iconic moments in the saga, but writing wise it is the defining standout.