Nice argument. But bacteria can infect organisms, even kill them. Can an Old One be "humanized"?
This reminds me of a character from the book series Laundry Files by Charles Stross. Basically the have this alien entity called "the souls eater" (guess what it can do) which a secret british govern agency evoke in the 60s and bind to a human body with some kind of binding ritual, after that they pass almost 10 years brainwashing him in human value, honor ecc. up to the point where he became one of the best secret agent of that organization, with a key role to save the world from eldrich horrors.
Sometimes we're a positive but most of the time we're not noticed unless we're a problem. A surplus of the wrong kind of psychic energy or not enough of the right kind could warrant a reaction just like taking antibiotics or eating a probiotic yogurt.
Like. It ain't going to be a direct one to one comparison, it's an analogy about hypothetical charecters...
the brainwashing didn't take. He's just playing along because he's afraid of bigger eldritch horrors, and the Laundry is his best resource for protecting himself.
Tell it from the point of view of the people, who start investigating aspects of the place they live that don't make sense, and at the end they come to the conclusion that they're in an ant farm being studied by the Old One and are driven insane.
The way I read the story, Cthulhu himself is positively inclined towards humanity - it says right there in the text that when he awakes he'll teach us new ways to revel and kill. We'll be his newest servitor race.
Well the old servitor race is still asleep in R'lyeh and will eventually wake up. So we'll more like be the "third wheel" race. Hard to accept i know, big Lulu is a "player".