Cazador is not a mad fanatic in a tower. That is a very important nuance. He is a pragmatic and patient strategist, who has spent centuries building his safety and influence. His diary is the journal of a conservative "shadow ruler" who sees the familiar, managed chaos (of the aristocracy and criminal underworld) being replaced by a new, unpredictable, and far bloodier reality. He is a parasite on the body of the aristocracy. If the aristocracy is destroyed (by the Bhaalists) or finds a new, more powerful patron (Gortash with his army of mechanical guards), Cazador will be left with nothing. He urgently needed to stop being a parasite and become an independent force, unafraid of any political changes. The ritual was to grant him that power.
Judging by these entries, Cazador clearly feared he had lost Astarion for good—his missing component for completing the grand ritual. Without him, the centuries-old plan would crumble. Interestingly, his anxiety about potential failure in the future struggle for power forced him to contemplate paths to absolute might well in advance.
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He began preparing the ritual long before new powerful factions capable of threatening his territory appeared in the city. For Cazador, this ritual was a trump card up his sleeve, a last resort for when a real threat would loom over him. And just when that day seemed to have arrived—Astarion vanished. Though not by his own will, he became the very stumbling block in the vampire's plans.
While Astarion was far from the city, Baldur's Gate itself descended into chaos. Bandit groups grew active, and the Cult of Bhaal began an open expansion (as we know, due to the rule of the triad—Gortash, Orin, and Ketheric, whose disjointed ambitions and power struggle plunged the city into lawlessness). Tension mounted, and Cazador clearly feared that the wave of violence would soon reach his lair.
He desperately needed to complete the ritual to obtain that very unlimited power and capability for protection and dominance. Until that point, he likely maintained his influence through local lords and nobles—via blackmail, intimidation, or by carrying out their "dark errands" without unnecessary witnesses. He must have understood: if these patrons found themselves a stronger protector or patron, he would no longer be needed. And for Cazador, a vampire forced to hide from sunlight, such a loss of connections would be a catastrophe. His strategy has always been to remain in the shadows, yet in close proximity to the sources of power, wealth, and blood—lords, countesses, and other elites, whose dark secrets he collected as leverage.
~This game impresses with its thoughtfulness. And I'm sorry that many perceive the villains from there as shallow buffoons.
I also mentioned other writers who might agree with my position @dynamicducks @carnivaley
