Well, as I can never decide on just one thing, I will review the most likely scenarios of the inevitable ending of humanity.
1. One situation that I thought of straightaway, was extinction through nuclear Armageddon. I don't believe that this is too outlandish, as the United States has already used two atomic bombs only sixty-seventy some years ago, and since then, many other countries have been granted permission to own, test, and demonstrate weapons of mass destruction. And even if countries have mutually agreed not to use them in times of war, we continually evolve and strengthen the atrocious repercussions of which can be obtained through detonation. We as humans have gone through many brutal, obscene times of war in the past, however with the prelusive invention of these new forms of missiles and bombs, we have only further accelerated our date of humanly departure. Here is a video a found of a Tsar bomb explosion in the fall of 1962:
2. If that is too unlikely for you, how about the worldwide permeation of a pandemic? A pestilence unbeknownst to the top minds of to-day, could, hypothetically, eradicate humans so feverishly and so unbelievably quickly, that by the time a treatment or cure can be manufactured, there would be nobody remaining to send it to. The Black Death of Europe in the fourteenth century CE wiped out a total of what is estimated to be seventy-five to two-hundred million people. It killed 30-60 percent of Europe's population, which thankfully at the time was only 450 million people. A plague of this magnitude in this modern day and age, would not be so merciless. In recent, we have developed airplanes, trains, cars, trucks, ships, etc. It would only spread quicker, and kill more and more people. One can only imagine how long it would take for the world to recover, if it ever would.
3. Now moving to perhaps a less serious yet more imminent threat, I find enjoyment in pondering our extinction or slavery as a result of a superior, technologically advanced, military force. Although these scenarios are only rarely taken seriously in movies, literature, and video games, I believe something of this nature could swiftly, within the realm of possibility, take place. Take Quake Wars: Enemy Territory, War of the Worlds, and many others as examples. Also, you must keep in mind, that just maybe, first contact between Humanity and extraterrestrials will follow the precedent of vastly different human civilizations meeting each other in the past, where typically the less technologically advances species succumbs to the more advanced, only because it cannot wage war and successfully come out.
4. Another idea, that is only less vicious, is the transformation of Humankind into a form of trans-human, bio-engineered race of cyborgs. I know, I know, you think I certainly must be joking, but if you think about it, many people, specifically amputees, are already sort of half machine through the assistance of prosthetic limbs. If it is possible to achieve this, then why hypothetically, could we not improve all of the faults of our fleshy, weak, bodies into a strong, resistant, mechanical chamber of the utmost excellence? Yet of course, I cannot honestly say that and not recoil in half disgust, as, placing humanity under a blanket of only technology, could be and can be and is seen by many as a more inconspicuous, yet more convoluted form of slavery.
Watch the following video to form your own opinion, or don't, I found it intriguing and thought provoking.
4.1. Something else similar to this I thought I should mention while I am on the topic of a less violent executions of humans, is the possibility of over the course of millions of years, Humans will eventually evolve into another form of species or hominid. Homo sapiens could fundamentally become one or more new different species who co-exist on the Earth together.