This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Armored Core
Anything relating to Human-Plus, which gives a pilot cybernetic upgrades... but generally damages their mind in the process, and it's successor programs in the later games. You will usually have to fight or work with the pilots that went through this at least once a game. Some are Laughing Mad, some are Creepy Monotone, but they're all incredibly disturbing to listen to. One example is a re-made mission from the first game featured in Nexus, where a pilot begs you for help in attacking a train somehow related to the program. At the beginning, he's overly anxious. Once the train is destroyed He turns on you, saying in a monotone voice: "Everything, yes. I must. Destroy. Everything."
At the end of Nexus, your character accidentally releases a mob of kamikaze drones that attack the world.. It cuts to your AC on the roof of an enclosed city, where you try to fight the entire mob by yourself. The screen slowly fades to black as your AC keeps calling out damage warnings.
RD and Chief being seemly insane in AC 5. RD is seemly killed earlier on, but he is turned on you and suffers a major personality change in a matter of hours and is ready to kill his friends for no explainable reason. Chief, on the other hand, is an AI and, therefore, can't really be killed. He lives for battle and keeps coming back after seemly dying several time. He laughs madly every time you defeat him. Even when his AC explodes and is on fire, he pulls down a concrete support column and rushes you. Many of his attacks are one-hit-kills if you contacts with you just right. He just seems so happy while trying to beat your face in..
You later know that Armored Core V and Armored Core Verdict Day are continuation of Armored Core For Answer. Let thatsink in for a second...
Expanded material puts the control system used in NEXTs as this in an And I Must Scream fashion- the system requires the pilot's nervous system to be hardwired into the control pod, or AMS. Which is good, because this allows the pilots to have near-zero latency in controlling their NEXT. They can just simply think of whatever movement or action they want their NEXT to perform, thus eliminating the need for numerous levers and switches and such. Which is also bad, because this virtually makes the NEXT an extension of their own body. This would explain how the NEXTs in 4 and For Answer seem to remain intact and in one piece despite being shot down. The destructive damage the mechs experience are translated into pain and felt directly by the pilots themselves. If you subject the NEXT to so much destructive force, you also subject the pilot to so much pain, which could make the pilot pass out or, in most cases, simply die inside the cockpit. Now, imagine what would happen if an active NEXT with a pilot inside had its arm blown off or be cut in half in the torso area. Or worse, have its head part ripped off...
The Bio-Weapons that occasionally show up throughout the series. The AMIDAs, in particular. Blecch...