Command & Conquer/Nightmare Fuel

Although the series is generally quite campy, It does have its many moments of scary things that will put you off from playing a game or two for some time. Most of these comes from the sudden realization about the various mumbo-jumbo in each series and imagining yourself to be caught up in one of these.

Tiberium Series
""What happens if you're exposed to Tiberium? Nothing good. If you come into direct contact with Tiberium, the green matter will start to fuse with your skin within about 20 seconds. You'll feel an intense burning sensation, similar to touching a hot pan or spilling acid on your skin. A full blown infection will manifest if you aren't treated immediately. Your flesh will begin to take on a glassy-greenish appearance as it begins to crystallize; eventually your internal organs will shut down as Tiberium extends rigid crystalline runners throughout your body. If you breathe in the crystal, then it will become embedded in your lungs. You'll lose the ability to process oxygen as your lung tissue crystallizes. Eventually you'll start coughing up blood and will hemorrhage to death.""
 * Tiberium, as summed up by one of the developers:

- Mike Verdu, "Living with Tiberium".


 * Tiberium is horrifying once you learn of exactly why it has it's properties. It's more or less a terraforming agent. However it leeches valuable minerals from the soil into a concentrated amount, growing crystals that can be easily harvested. The gas is toxic to most lifeforms and generally devolves them as well, but is easily contained. Bordering on Fridge Horror, you realise that this thing is made to capitalize on greed. Every single society will fight tooth and nail for control of this stuff, as there is literally no better way of extracting valuables. They will also purposely cultivate fields of the stuff, because it's far more easier to plop a crystal in the ground than digging through a quarry. But it leeches away nutrients for growing food and degenerates (and poisons) local fauna. This ensures that whatever species left when the Scrin arrives is completely crippled, with but pockets of unmutated stock left and mostly worn out from war and starvation.
 * Visceroids in Command and Conquer. They're disgusting enough in the RTS games, but the Renegade FPS gives them a whole new level of scary, seeing as you can see how disgusting they are up close and personal.
 * Not enough? Then go for the Nod Mutant army, which is anywhere from dumb Initiates to clever Acolytes to really tough Templars.
 * Buzzers. Swarms of thousands of tiny Scrin bladed organisims whose sole purpose is to flow through the cracks and gaps in your armor and shred you to tiny pieces.

Red Alert Series

 * Soviet Terror Drones. They could make instant mincemeat of most infantry, are annoyingly fast, and can burrow into vehicles to mutilate their crews. The fact that the latter is only shown on screen as said vehicle rocking only makes it worse...
 * Terror Drones are actually dismantling the vehicles, the crew just dies from the resulting explosion (that's why you have to repair them to get the drones off). However, note that the Drones were designed with Dismantling Vehicles in mind and was only suppose to do that. The butchering of infantry was when the Soviets realised the same tools used to cut through metal and sever cables can be used to do the same to human flesh and limb.
 * Getting erased from time.

Generals Series

 * Death by Anthrax intoxication, Nuclear radiation, burning alive, etc.