Hereβs a wild theory inspired by @emeraldinerosefaedragon.
The reason Alex might be so damn soulless might be because Gabriella is his maker.
A vampireβs maker can determine what kind of vampire they will become. While some inherit their eccentricity, others may inherit their strength and prowess. And still others may inherit their viciousness and penchant for extreme cruelty. But how they are made also determines their vampire nature as well.
While makers often make their fledglings out of love, devotion, and/or obsessive affection for them, not all vampires are made that way. Some are made with malicious intent, methodically to create servants, or forcibly put in a position to make them (e.g. Jasper, Alexander).
Checkers is a revenant made by Jasper who is vicious and only lives to cause destruction and destroy his makerβs enemies. Daciana was made a revenant by unknown means, but was decaying by the time Louis and Claudia found her and it was too late. Doris was forcibly made by Alexander into a weak vampire by the Talamasca and lived in isolation to hide her entire existence.
While some vampires are unable to handle the madness of immortality upon their turning, others who are made without love are also bound to be born with some form of defect. Maybe Alex isnβt an exception to that.
Unlike TC and Salamander who were (presumably) made by Lestat with love, care, and affection, Alex was made by Gabriella with no love. Sheβs neglectful, cold, and only made him out of a twisted sense of entertainment and for her own selfish goals. Not to mention the ongoing grief he feels from losing Larry. All of it probably had a negative effect on his turning which made him into what he is now.