Artificial Intelligence.

Hate to be that guy but global warming will almost certainly be our downfall. We're pretty much past the point where it can be fixed, and by the time we make changes parts of the world will be flooded, billions may die, and we're pretty much screwed.

Lack of tolerance for others. Lack of understanding of others. Lack of willingness to look past what bothers us about others.

The conservative movement. Conservacrites allow bullying and when the target stands up for themselves, they blame the victim.

Narrow-mindness and double-standards of living which every metropolitan guy pursue will be the ultimate downfall

The common delusions borne from the emergence of our consciousness... the artifices of significance, meaning, free will, morality, etc.

Our nation will most likely not fall directly to outside attacks of any sort. At the moment we remain militarily powerful enough to defeat any other nation. Our cyber vulnerability will not be our undoing, because we are not any more vulnerable than our enemies.

Our potential for continued drift from the things that make us strong will most likely be our undoing. Our potential for electing leaders and lawmakers who lead us away from the principles of governance written into our constitution, remains our greatest enemy. Divide and conquer, will end us, unless we find another great leader to unite us.

Our downfall is the day where anything and everything is accepted. The day where rules aren't applied and people can do what they like because it's "their right" and "they're just being themselves". The day where pedophilia and bestiality will be legal (practically already are) and people can do what they like because "we can't judge them". That will be our downfall.

Humans will destroy the Earth with nuclear war. It's so sad.

I have no idea what will lead to the downfall of humanity.

I know it won't be global warming (at least not the trendy one), nuclear war, AI, conservative movement, "a day where anything and everything is accepted", lack of tolerance/understanding, or whatever Jace said.