Summary: "What scares the monsters: thing(s) NOS-4-A2 is, or would be, afraid of?" (Requested by Rook of To Infinity and Fandom)
If you could have asked NOS-4-A2 what haunted him at night, he would have answered "Nothing." What would he have had to fear? He was a robot, not prone to irrational fears like organics were, and any rational fears he should have as a robot were overcome by the knowledge that he WAS those fears personified. He was the threat of deactivation, often permanent, due to his attacks. He was a virus, corrupting the files of those he bit into, changing their programming to serve him and ONLY him... But, there were those that defied him. The little one had somehow regained control of his mind and defeated him... Defeat... That, in truth, was the concept that drove NOS-4-A2 to the edge in fear. He was never programmed to accept defeat. He was to win at any cost. Yet time and time again he was defeated, each more pathetic than the last. Stopped by a little girl. Beaten by veggies. Blasted off a planet after doing hardly anything! Each defeat played on repeat in his memory banks, crippling him in his times of rest and causing an obsession to win the next time at any cost, just as he was programmed. That was why, in his most spectacular plan yet, he used XL and the Wirewolf to try to take over. It was a risk, to take over Planet Z and then mutate the organic world, and if even one thing went wrong he would likely be defeated once again. It was a desperate bid to win at least once.
The spike through his chest didn't hurt nearly as much as the sheer terror through his mind as he realized that this was his final and most horrible defeat.