There are a few professions that just scream "evil." Mad scientist, corrupt politician, creepy motel owner, art critic… wait, what?
Meet Ellsworth Toohey, the world's most evil art critic. Don't let his mousy exterior fool you. This guy is no kind of good. He resents anyone more talented than himself… and in Toohey's case that means the majority of the population.
He's not talented and he has no sense of humor, but he is manipulative, scheming, and oddly charming. He's got a little of the devilish charm of Faust's Mephistopheles, and a lot of Mephistopheles' cunning.
So why, with this stockpile of Machiavellian tendencies, does he decide to be an art critic? Well, it's important to remember that this book is in large part an Objectivist treatise and is chock-full of symbols and allegories. In the Fountainhead, architecture (and art in general) is really a metaphor for individual creation. As an art critic, Toohey is positioned in opposition to individual creation and, indeed, to individualism. What better way to attack the individual expression of Roark's awesome architecture than as an art critic?
Toohey's philosophy is the polar opposite of individualism. He's all about collectivism; he believes in the crowd. Unlike Roark, who is all about life, liberty, and the pursuit of happiness (Roark is presented as being as American as apple pie), Toohey rants about sacrifice, denial, the virtues of being miserable, and the unimportant nature of the individual.
Toohey's ideal world is a creeptastic hivemind:
"The world of the future. That's what I want. A world of obedience of and unity. A world where the thought of each man will not be his own, but an attempt to guess the thought of the brain of his neighbor who'll have no thought of his own [....]" (4.12.93)
Toohey is the anti-Roark. Roark is all about the power of the self, and Toohey preaches the importance of collectivism. Roark doesn't try to influence anyone, and Toohey uses his art critic's platform to sway the minds of the populace. Roark thinks that striving towards greatness is, well, great and Toohey is big on mediocrity.
If this sounds less than subtle, it's because it is less than subtle. Rand hammers her point home with a big ol' iron mallet when it comes to the characters of Roark and Toohey. We can imagine her saying, in a Tarzan-style voice, Roark good. Toohey bad. Roark good.
Roark may be awesomesauce and Toohey may horrible, but they have one thing in common: they're both completely two-dimensional characters. Just as Roark springs fully formed into the world of The Fountainhead as the ur-Objectivist, Toohey is always a sniveling little herd-mentality worshipper.
And he doesn't change throughout the novel.