Which is the most eminent and probable/possible one? Explain your reasoning.

AIDS??

Hm. Well the world's kinda in too good of shape for people to die off anytime soon. I hope a meteor does the job cause' I don't want to be around if my guesses come true.

1. Herd Poisoning
It's a condition studied and mastered by Hitler, where people stop thinking for themselves and mindlessly go along with whatever the majority chooses. Because of herd poisoning, people fall for propaganda, never question anything, never fact check what they're told, don't use logic, are hypocritical, and so on.

Just take a moment to flip through television. Watch how people "debate," read posts on hate sites, watch how science is held back by superstitious beliefs, and recall how people have destroyed religion. Hell, just look at how Hitler made people believe it was okay to round up a bunch of Jews and kill them.

Now, stupidity and ignorance from herd poisoning always leads to what COULD be the end of civilization. Herd poisoning may be little to others but to me the root of the problem is the major problem.

2. Squandering of Resources
In industrialized nations, all people do is consume, consume, consume. People waste unnecessary resources. Resources will begin to run out, starvation will rise in third world countries, and inflation happens. Simply put, when things run out completely, people will die. People might kill each other for resources and eventually die out themselves.

3. Overpopulation/Too Much Reckless Sex
What?! I thought more babies equals more lyfe!!11 Hear me out. When people continue to add more mouths to feed, it leads to point 2 much faster.

Not to mention, people want to act retarded and forget that family clinics give condoms for free. As Mico said, STIs will spread, and sadly babies will be born with some and continue to spread these infections.

4. Bystander Effect
If people continue to leave it to someone else to uphold morality, gang violence, terrorism, and aggression will go on unchallenged.

Edit:
Here is a video showing point 4. It's graphic and speaks for itself.
http://www.cnn.com/2008/CRIME/06/05/ign ... nnSTCVideo