UFO: Enemy Unknown or X-COM: UFO Defense
Developed by Mythos Games, MicroProse Software. Published by MicroProse, Spectrum Holobyte. Released in the summer of 1994 for PC.
Alien attacks and humans being abducted start to become a concern for nations around the world. Leaders around the globe meet in secret to discuss how to solve this dilemma, resulting in the creation of the Extraterrestrial Combat (X-COM) group. X-COM has the player managing global headquarters and progressing in an overall conflict against alien invaders, alongside turn-based strategy missions that depict conflicts with the aliens directly.
X-COM Enemy Unknown had a dark ending cutscene if the player failed to stop the enemy invasion. The nations of the world concede to signing treaties with aliens, promising peace. The leaders of humanity face betrayal from the aliens. They proceed to exterminate the human race and drain Earth's resources.
The Playstation version of X-COM goes a step further with envisioning this frightening reality. The scene opens with a UFO flying over the United Nations building. The leaders of the world are trying to form a treaty of surrender, when aliens burst into the room. A look of defeat comes across the faces of the leaders as they know they've lost the battle. The alien blows their brains out and spatters it all over the negotiation table.
X-COM is scary because it could really happen. Scientists at NASA eagerly study the stars and cosmos to search for extraterrestrial life, but I'm worried they'll eventually actually contact a race in a galaxy far away. Humans on Earth don't know what's up there in the stars and the looming answer to that could be a harbinger for the planet's destruction. Space is a vast expanse of mystery, there really might well be aliens out there wanting to destroy the human race. Don't think about it too much.