Hallo there! I'm currently creating a campaign that contains a country that has guilds divided into the four element. The twist I want to have is that one of them will turn evil and wage a war with the others with a secret army. Trouble is, is that I can't decide which. Fire would be obvious, but it's a bit of a cliché and I want to catch my characters by surprise by who turns evil.

Thanks for any advice you give!

Vote anyways if you don't comment, just asking for your opinion.

You should go primordial, there are so many options to choose from and they are beings that have power equivalent to gods, so you have a villain for the whole campaign right there. There are primordials like Vezuzu the Burning Mountain, a volcanic primordial, you get earth and fire in one. Also primordials don't necessarily have to be evil, in fact most are neutral, but they have a bone to pick with the gods who they feel invaded and stole the planet they shaped. 

There are even primordials like Rorn the Blazing fury (can turn into a mini star) that have mortal followers that they protect. Look up d&d cult of the dawn. 

Awesome! Thanks for the input. The campaign I'm making does have a custom primordial god that has a Chaotic evil alignment. This will fit perfectly! I'll have to create a secret fifth guild though.

Anyone else is welcome to put in a suggestion. 

I vote Air, not only to make it the opposite of Last Airbender and all but you could have the Air guild frame the others by using strong air manipulation to move other elements to cause problems. Hopefully your players won't know who to trust at first and will think everyone is turning on each other so they will hopefully be shocked when they find out it was just the Air guild the whole time.

Start a fire by having gusts of wind toss extra kindling into fires to make them go out of control.
Manipulate water at the shorelines to make giant water vortexes with a tornado that sucks up water.
High powered gusts of wind to somehow cause a rock slide or something.
These examples are pretty bad probably, but if you can things like that but better it may work out.

You're thinking on the small scale with air.  Changing prevailing winds can cause cold snaps and terrible blizzards, droughts and atrocious heatwaves.  Blow ice floes from the north blocking harbors and strangling shipping routes.  Becalm the winds on the seas stranding ships in the middle of the ocean, at the whims of the currents.  Wind has a great potential for mucking with everything, too much rain, not enough rain.

All great ideas and perfect for Air villains who don't care about getting caught.

The goal of my examples was to show ways they could frame the other elemental guilds so they can play innocent as long as possible, causing the other guilds to fight among themselves before they do get caught by the PCs.

If you want to go subtle then it is just adjusting wind currents to the other countries so they get some unseasonable rain during harvest season and other subtleties to weaken the other countries, they could even do it to themselves if they had prepared ahead of time by stockpiling large quantities of foodstuffs.  Maybe even allow them to sell their goods at exhorbitant rates to the other kingdoms.  Economic warfare can be a good opening salvo in a big war.  Generates money to use for hiring mercenaries and the likes.

Just to clarify, I'm running the four elements as guilds, not countries, but they do have their separate regions in the country itself so the air control does still apply.

I think there must still be areas where each guild is the dominant group.  Or at least the most influential Guild.

I see another vote for water.  How would they go about being the bad guys.

Probably have to do with shipping up and down rivers and via ocean/sea routes.  Perhaps they create water for some desert regions for irrigation.  What shape are you thinking of using for your world, or have you drawn a world map yet?  

How the whole world is, is that it's shaped like an infinity sign with a giant chasm separating the two circles. This creates the two kingdoms in my setting. A chaotic one to the west and a sophisticated one that includes the guilds to the east. I haven't been able to exactly figure out the geography, but just to mention there are plain, forest, tundra and mountain environments. I've even included an archipelago kingdom,.

So something like a peanut with a giant deep rift running down between them, related question does the rift go all the way down to sea level or does it go even deeper into the underdark.

So something that could be fun for the water side being the bad guys is they could be recruiting from the uncivilized continent (I think tribes of orcs, goblinoids, dwarves humans goliaths and similar thing from there) and maybe as a later event they are investigating further into the rift than anyone has gone before (which is bound to stir up some trouble they are not ready for).  Granted the exploring the rift could be a major feature for whichever guild its the antagonist.

Something that could be fun for your players is if you make a map have the 'known' world well dilineated with states or countries, with the other continent having sections listed as HERE BE GIANTS,  REALM OF THE DRAGON LORDS, THE BLOODY TRIBES, and other vague ideas of what is truly there, which might tempt the players to adventuree there at some point either looking for allies or to shut down recruitment by the bad guys.  For some added interest you could have an underlying concept of two gods (twins, brothers, sisters, siblings) created the two continents.  Might make for some inspiring lore to work off of for yourself.

Keep giving us details of your plans and what you like and I will try and give you new directions to think in.

Ok so here's the lore behind the rift.

It was originally the land of three dwarven fortresses known as the Borderholds. A dark cult created a giant portal just outside of these fortresses that brought the nine lords of hell into the material realm. Adventurers and warbands from the western land, magi and sorcerers from the eastern land and the dwarven machines of the fortresses attempted to send back the lords but to no avail. This was until the dwarves sacrificed themselves with humongous bombs laced with magic that dragged the lords with them back to hell, creating the rift.