Santiago loves to persecute, he loved torturing his victims onstage, he’s a manipulative bastard! Even better when the victim is a particularly lovely creature like Louis ;]

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Yeah, I suspect and there is a secret fanfic that supports this theory that Santiago considered himself 2nd-in-command and maybe more to Armand, and he felt himself very usurped when Louis came into the picture, and had it in for Louis from the first moment Armand made heart-eyes at Louis.

Santiago was also a Vampire of the Rules, as long as following them worked for him. Louis had been an active accomplice in attempting to kill his own maker twice (THE cardinal undead crime); attempting to keep it a secret doesn’t engender trust in such a member, in fact it provides a good reason to feel threatened by them.