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Nightmare Fuel: Mobile Fighter G Gundam
The Devil Gundam itself. Not only is it Satanic in name and appearance, it also infects other people and turning them into robotic zombies.
Earth under attack by the Devil Gundam in its Colony Mode. It destroyed a lot of cities and whatnot, offscreen deaths could have soared higher and higher in count.
The Devil Gundam again! Aside from the above, this could also make people fall under its command. And destroying it would be impossible even if it required a massive amount of Gundams and other mechas around only to be destroyed by it and its only death is The Power of Love.