I think I read somewhere that the Keyblades were for fighting over the light, and after the World fell, to protect the light.That said, it wouldn't surprise me that the Keyblade was originally a weapon developed by the Darkness in order to control the Light, easy navigate through it and what the MoM did was modify the Keyblade to fight Darkness, and without realizing it, generations of Keyblade Wielders and Masters have been using a weapon that was originally intent to destroy the light.
About Darkness being a force of nature,
like King Mickey and Riku said, either can’t exist without the other, so it makes one wonder why it’s to be feared. Riku answered it’s because of what’s lurking inside it. By itself, without sentience, darkness has proven itself a benevolent utility, such as Kingdom Key D, resurrection (though this was sadly applied only to villains so far), synthesis materials, abilities like Dark Firaga and others. I would suggest Guilt, but it’s subversive and hinted to actually be harmful, and might be the reason player have a dark Chirithy.I guess my problem with "Darkness is literally darkness itself" is that I still believe the idea that darkness and light are just forces. Darkness is a really dangerous force, so it's usually used by people who are reckless or let themselves lose out to it, and thus why the antagonists use it. But I don't know if I can like Kingdom Hearts if it comes down to "darkness is literally evil and light is literally good". But I do agree the Darkness characters are the "monsters" from his past that MoM spoke about (as I've guessed in my thread about the unversed symbol).