Sometimes, reality checks are too painful if you are too invested in the fantasy, I guess...
I've tussled with these same types of thoughts. How do people get this way? They obviously look at the world through a different prism, but how/why does it happen? We may never know. But it is fun to watch them wail and gnash their teeth!
It happens step by step. They didn't start off that crazy, but by building up the lies, listening to and believing their rhetoric, they create a shared reality that is the space they work from.
I mean, when you believe truth is relative, it's easy to avoid fact checking...what you feel becomes more important than what IS.
Of course, even if you don't believe in gravity, if you drop a 15 lb. bowling ball on your foot, it will still hurt....