Since early childhood, Alec had the idea that he wasn't exactly human. This delusion was brought about by hallucinations and vivid dreams, as well as his mother's abuse and his general alienation from everyone around him, other than Bertrand...although sometimes he found it hard to connect to Bertrand as well.

As Alec got older and embraced his worse tendencies, he started leaning into the idea that he was better than everyone else naturally, and that he deserved to get whatever he wanted and whatever he could achieve. This sense of superiority became entwined with his previous delusions. However, this didn't bloom into a full on god complex until after Alec's exile in Bali and his induction into the criminal underworld. The power he had access to, not to mention his drug use, sent these previous tendencies into overdrive. The more he got away with the worse he became.

And then Bertrand died, and Isabelle became the Messiah through which Alec's god complex could be realized. Alec was free of his most shameful vulnerability in the form of Bertrand, and here was this being Alec had created over which he had total power. So to answer your question, it was gradual over 30+ years! Far from built in a day, but it wasn't really "finalized" until Bertrand was dead. I plan on writing a short fic of Alec at Bertrand's funeral with baby Isabelle to illustrate all this.