Birdman wrote:So Cthulhu can't induce madness in people? At least, not willingly or as a guaranteed power?
In some interpretations, yes. In others, no. The Cthulhu mythos is a hodgepodge of stories by a bunch of different authors, along with many other stories in different mediums.
There is no set canon.
In some versions, likely including Lovecraft's original, KaijuKing42's assessment is correct. In other versions, Cthulhu is in fact capable of inducing madness in people willingly.
I say likely including the original because the original tells us next to nothing about Cthulhu's abilities. Which is why most people rely upon the Cthulhu Mythos for his abilities, but like I said, the Cthulhu Mythos is confusing because there's no singular canon, yet no distinctions are made between the canons. Also, most stories don't even have guys like Cthulhu come out preform feats of great power, because that would be the end of the story, because it would be the irreversible apocalypse, in just about every case.