Elmo, though cute, has no redeeming quality in his personality or action. Normally this wouldn't directly be a vision of Satan, but there is more. And besides, it is well known that the Supreme Evil Being is supposed to be beautiful... perhaps this is his son and he is going through that early stage. Anyway...

Reason 1 - Speech

Elmo is not a role model for the speech developement of children. First is the constant referral of himself in the third person, as in, "Elmo so happy to see you!" and, "Elmo loves his goldfish," and, "Elmo would like to introduce you to his friend, Mr. DeNiro!" (Note: Deniro once played Satan... wink wink) and of course, no one speaks in the third person except for hoodlums, criminals, and (insert celeb here).
The next thing about his speech that is derivative is that he consistantly leaves out the existence verb, to be. This is in such sentences as "Elmo so happy to see you!" and "Elmo going to play with Zoe now." This encourages people to embrace Nihilism and show that it is better to not exist, consequently causing them to become cruel and mean and stuff (Not that all Nihilists are like that, just the ones I know) and thus become his tools of destruction.

Reason 2 - Cloning

This reason is a bit more difficult to explain, but it mostly involves the fact that there seem to exist several Elmo's. The current theory is that they are constantly cloning themselves. The process is repeated every episode to constantly create more and more Elmos until he takes over the entire show. I have seen a maximum of 7 Elmos on screen at once. Once there are enough of them, they will, I believe, proceed to latch on to each child watching the show. Every child will grow up with an Elmo parasite in them, which will eventually control their bodies for the gain of the Great Beast.

Reason 3 - Repetition

After watching four or five episodes of mindless Elmo drivel, I have discovered that his shows are the exact same thing. First, he sings the ridiculous theme song. Then, he announces what his Evilness will be talking about today. A short clip with mind-numbing Kazoo music invariably follows, after which Elmo talks to Mr. Noodle. There are some more clips with similar "music", after which Elmo will turn on the TV to watch the "insert subject here" channel. Then he sings the "insert subject" song which is a high-pitched, squealing version of jingle bells with the current topic repeated over and over and over. The point being, mindless repetition of the EXACT SAME THING will drag children into a thoughtless, blank stare at the television, at which point The Lord of the Flies will proceed to take over the mind, and subsequently body, of the unwitting viewer.

Of course, there are many other things that point to Elmo's hideousness, such as his uncanny understanding of animals, particularly his goldfish, and his mysterious moving furniture that obey his every whim. Even Mr. Noodle is his toy, because Mr. Noodle is basically a man engineered with no brain so that he will do whatever they tell him to without a second thought, and if he gets it wrong, well, it's time to teach him how to do it RIGHT... "Mr. Noodle, strangle that person with a piece of piano wire! No, no, not like THAT! Around his NECK! Ha ha HA HA HA HAHA HA HA!!!" So please, join my crusade against Elmo! Help destroy his army of darkness and save innocent children from becoming his mindless, toiling, eternal slaves!