Turning over in my mind how Astarion and Cazador are mirrors of each other and it's sooo interesting. Like, Astarion when the PC meets him is on a kind of precipice. The PC can either encourage him to continue on the path he's on, resentful that other people might get help when he didn't and willing to do anything to make himself feel safe (might makes right, etc.), or can push him back in the direction of being "good," caring about others and keeping his worse instincts under control.
Astarion spends large chunks of the game castigating Cazador for his senseless cruelty, for his selfishness, for his derangement. Astarion also frets about being or becoming a monster. If the PC tells him during their first meeting that the parasites will turn them into mindflayers, his response is a dejected "Of course it'll turn me into a monster."
And yet, the moment Astarion grasps that he might seize control of Cazador's ritual of ascension, that he might sacrifice his vampiric "siblings" to obtain the power Cazador sought, he is immediately sorely tempted, even eager. When it becomes clear the ritual necessitates the sacrifice not only of Astarion's six spawn siblings, but also the seven thousand victims Cazador has been collecting in the basement for centuries, many of whom Astarion himself delivered unknowingly to their fate, Astarion hesitates only slightly in his desire to complete the ritual himself.
If Astarion is ascended via Cazador's ritual, he almost immediately follows in his master's footsteps. Some of his first dialogue after ascending is about feeling that every other living thing wants to serve and be controlled by him, the vampire ascendant. If he follows Cazador's path, he becomes Cazador.
Astarion was tortured for centuries into the person he is in the game. He admits to the PC that he barely remembers his life before being a vampire. The person he was before Cazador entered his life is almost entirely lost to him. After centuries of being on Cazador's leash, Astarion barely has a thought that doesn't revolve around Cazador. Minthara correctly points out that as long as Cazador lives, Astarion will never truly be free, because so much of his mindset continues to center his old master. Whomever Astarion might have been if Cazador had never touched him is a mystery, even to Astarion himself. He is now what Cazador made him, but the PC can encourage him to grow beyond that.
By choosing to ascend, by following in Cazador's wake, Astarion essentially rejects the chance to become something else--he accepts that he is what Cazador created and he will become what Cazador wanted to be. If an ascended Astarion is permitted to leave the Szarr mansion alive (ie. the PC doesn't turn on him with the Gur), he revels in his power and in the Reunion Party scene brags about being the "puppet master" of Baldur's Gate. If the PC is in a romance with him, he may agree to turn them into a vampire spawn, but refuses to make them a full vampire, for vague reasons that almost certainly amount to not wanting to have someone around who might even possibly compete with his power.
Astarion thus shows that he has the capacity to be just like Cazador.
By extrapolation, then, Cazador may have once been like Astarion.
The team learns by exploring the Szarr mansion that Cazador was turned by a vampire named Vellioth, who treated him horrifically. Simply by interrogating Vellioth's skull--which Cazador keeps in his bedroom as a gruesome memento--you can hear about how Vellioth murdered Cazador's friends in front of him when he reached out to them, made him spend eleven years impaled as punishment for a failed attempt to kill Vellioth, and how they both laughed when Cazador finally succeeded in killing his master. Clearly, Cazador is what Vellioth made him, down to the scroll of rules he recieved and then imposed on Astarion and the others, and compared to his old master, he may even view himself as lenient with his own spawn. (Astarion can remark that "even his precious rules" weren't something of Cazador's own invention, which shows how much of Cazador and particularly of his presentation as a vampire is made up of Vellioth. And yet--if romancing an ascended Astarion, he almost inarguably delivers the same set of rules, albeit phrased a little gentler, to the PC. Thank you to corgiteatime for the link!)
Who was Cazador before Vellioth? We don't know, and Astarion doesn't either, but the hints we gather in the Szarr mansion suggest Cazador's journey to his present monstrous state was not very different from Astarion's own path to the climax of his personal quest.
(Pure speculation, but Astarion suggests Cazador took special pleasure in tormenting him among the other spawn, and if Cazador saw something of his own pre-vampiric self in Astarion, I think it tracks that he might then in turn feel particularly hostile towards Astarion, perhaps resenting this reminder that he used to be something else, before Vellioth.)
Astarion lives in perpetual fear. Fear of Cazador, fear of being controlled, fear of being hated and despised by those around him, fear of having to face up to his own actions. His desire to seize on the ritual of ascesion is out of a desire for safety--one of his lines about wanting it is about how no one will ever control him again. Is Cazador's desire for it dissimilar? Rifling through his corresondence in the Szarr mansion shows he is eager to control Baldur's Gate and grow his own power, but of course he wouldn't admit to rival vampires that there is buried in there any desire for his own safety. It is not unbelievable that even centuries after Vellioth's death, even as a true vampire now himself, Cazador is still chasing some sense of safety. It is also a preview, I think, of an ascended Astarion's life: the fear that will never leave him as long as he continues to walk in his master's shadow. Astarion believes that ascending will excise that fear--but I'm sure Cazador believed that becoming a full-fledged vampire would make him feel secure, and here he is chasing the ritual of ascension.
Astarion and Cazador are two sides of the same coin, two stones in the same path begun by Vellioth (although, truthfully, probably even before him, by Vellioth's master and that vampire's master, and so on). Astarion can continue down that same road, chaneling the abuse he recieved from Cazador into his own cruelties, or he can end that terrible lineage by refusing the ritual, by refusing to become what Cazador wanted to be. Cazador represents the monster Astarion can truly become, the worst of Astarion's impulses and instincts, but when the PC meets him, Astarion is not yet too far gone--he can be turned away from this. For Cazador, it's much too late.