Don't you understand how deep and dramatic it feels to truly understand that the world doesn't meet your made up standards? Nothing's better than feeling like a tortured soul.
Kind of like when people joyously proclaim that they're an asshole, or a notable member of an MMO I play who loves to go on about how he's a 'sadist and doesn't give a fuck' - crowing over the worthless masses that will never be as enlightened as themselves. Alternatively, there's also a good feeling to being a 'disgusting fleshsack' writhing in the mud alongside the rest of the world, tragically lamenting how dramatically horrific everything is.
There's also a lot of egotistical nonsense to the argument - 'The world isn't perfect and people don't behave the way I think they should (OPTIONAL ADDITION: and I'm not the glorious paragon I want to be/think I should be (OR: think I should be venerated as)) so humanity should be wiped out. If I can't have what I want then everything has to go'. In short it's like a child getting something for Christmas when they wanted something else and immediately declaring the present worthless and trying to destroy it.
And come to think of it, who is a single, often self admittedly worthless (oh the humanity...) person to condemn an entire species because they didn't get the perfect world they wanted? One question - is humanity, as a species, a generic hero from a book, or are we real? We're real. In a (generic) book, the hero is sympathetic, he gets what he needs handed to him by Plot (magic sword, old wise mentor). Things just happen for him - even at the start, does he set out against all odds or is he pushed out the door by a burned down hometown? Oh he has challenges, but even they are served up by the plot for him to overcome and learn from. Essentially naysayers seem to think that humanity's future should be handed to us on a sliver plate. Rather than working to eradicate disease and poverty, we should magically transform into a disciplined army of doctors and scientists, and from there into renaissance men and philosopher-princes to populate our new utopia. If humanity is a person, then it's a conflicted one with plenty of flaws. If it works at improving them, things will get better. But the doomsayers seem to think we should be the hero of a badly written book, a perfect, flawless Mary Sue, and if we don't default to this ideal state then clearly we should wipe ourselves out, unworthy bastards that we are.
