There is the extremely unlucky event whereupon the Earth may drop into a black hole and be wiped from existence in a blink of an eye. Perhaps the chances of drifting into the path of a singularity are small, but the possibility surely exists. Nothing in science says this can’t happen.
At some point the center of our galaxy will implode on itself, releasing a gamma ray burst that will irradiate and destroy all life on all stars in the Milky Way. It’s a sure thing that this will be the eventual death of all life in the galaxy. Allegedly, this isn’t supposed to happen for an extremely long time. What do those scientists know about collapsing super-massive black holes anyway? For all they know, it could take exactly 3 more stars and the center of our galaxy would overload and kill us with gamma radiation.
A huge meteor or comet could smash us all to bits. It will happen sometime, and maybe sometime really soon. It might happen right when you stop reading this sentence. It happened to the dinosaurs and it is bound to happen at some point simply because the probability exists.
An advanced alien race could have engineered our planet for themselves when their home planet dies. Finding us in our greed and war crazed state, they eliminate us summarily with Super AIDS. Aliens are far too advanced for any other primitive kind of weapon, including a nuclear bomb. To them, killing humanity would be no more than a quick game of Pandemic 2.
The Universe could be a simulation from a computer-like system in a more complicated universe. Some scientists and philosophers give this a moderate likelihood. If this is true, then within that likelihood exists the chance that the computer-like system could be destroyed, or our process ended. Some weird super being could just ctrl+alt+delete and everything that ever was would be data lost forever.
The chances against a nuclear holocaust are very slim. In fact, it rests completely upon the chances that humanity dies in one of the aforementioned disasters first. A nuclear weapon could feasibly detonate at any position on any part of the globe and at any point in time. Your house might be ground zero. In fact, you better hope your house is ground zero because in a nuclear apocalypse the dead are the only people who will not suffer. Of all these events, this is by far the most horrific and most likely.
With the combination of all the probabilities for different known cataclysms, the Earth is on its very last leg. Each day is one day closer, and one way or another, the day might come tomorrow.