Just like everything else in such a symbolically loaded book, it all depends on interpretation. Dragons have typically been associated with the Antichrist mostly on the strength of the book of Revelation (specifically Rev. 12:1-9, among other references). However, there''s considerable speculation that John was writing symbolically about the Roman Empire in that passage, and so the question may hinge on: Why was a dragon used as a symbol for something so evil?

Dragons are a very powerful Other, instantly recognizable as something alien to humanity. In a religion so centered on humanity as Christianity, this placed them naturally in the role of enemies. They may have been associated with the enemy simply because of the strength of their symbolism and mythology and their universality.

Of course, not all references to dragons in the Bible portray them as evil. But in general, Western thought has been to condemn them -- unfairly, I say.