Yesterday I read a book that featured some excellent characters and a most intriguing plot. One of the reasons the story worked so well was because, in a wicked twist revealed toward the end of the book, one of those characters who had appeared throughout the story as an heroic figure turned out to be both an antihero and an antagonist, albeit unwillingly.  

An antihero is a character who appears to be a champion of the cause but lacks the usual heroic qualities one might expect, such as bravery or honesty. An antagonist works against the hero or protagonist and their efforts to resolve the conflicts and complications of the plot. 

Interestingly, antihero and antagonist both have roots in the same word element: anti. 

To be anti-something is to oppose it in belief, thought and/or action. 

The prefix anti- is very old, dating back to Ancient Greek and, even before that, Sanskrit and Proto-Indo-European. It means against, opposed to, or opposite of. It can also mean in front of or before.

From the Greek, it made its way into Latin, and thence into Italian, Spanish, English and French. That makes it a prefix that is very widely understood around the world, and one that is attached to many, many words to add a sense of opposition or contrast. 

Thus, although anti-masker is a quite recent term and antichrist is a designation as old as the Gospel itself, we understand both equally well because of the simple clarity and strength of the anti-  prefix. 

Sources: 
Etymonline
Macquarie Dictionary