Of course we're doomed. It's only a question of when and how. The dinosaurs went in a spectacular genocide caused by something not of their making -- perhaps a meteor strike or the all-enveloping dust from a monumental volcano. We, too, could be victims of that sort of cosmic bad luck. But, within the present century, we might well be snuffed out by our own profligacy, greed and stupidity. We, the most versatile species thrown up by evolution, are the first that has the ability to destroy itself.
Some of the dangerous consequences of our own actions can be specified with increasing certainty. Population to hit nine billion by 2050; global warming of more than 2°C almost inevitable; food prices to double within 20 years.
But the greatest threat comes from the human reactions to these challenges. With access to nuclear weapons and perhaps, soon, synthetic diseases, a psychopathic leader could destroy us all. And when it comes to predicting the likelihood of such cataclysms, the normal rules of chance break down. You could say that Hitler was a one-in-a-billion event, yet the force of his personality and the social crisis that he was able to exploit made his rise to power much closer to inevitable.