Why can't he/she just be named "One Being?"

Well, seems to me - "One Being" sounds too... Harmless. And too obvious.

And who the hell would have given The One Being a name? Himself?

A name is a tool so other people know what to call you. If you're the only thing in the universe, what do you need a name for?

Well, Armageddon never happened, it cannot name itself, yet people call it - "Armageddon". This thing can be named by people - according to visions, prophecies.

...and it was named The One Being.

So, It appears that there's no difference - to call it the One Being or something else.

First of all, I don't think the One Being should be regarded as a he or a she and I think it's actually kind of silly to give it a name that can translate into "evil"...especially since the One Being itself really isn't necessarily evil.

If it's to have any kind of name, I would go for Brahman. I think that would be the most suitable...although I'm certain there would be people who would get confused with Drahmin.

Yeah I agree they should be just called One Being. That's what I did when I created the One Being in Armageddon. One Being sounds good enough.