Why bother? Why scare yourself?
For one thing, these are memorable stories you will enjoy and won't easily forget. For another, humanity does face catastrophes of various levels of scope and various gradations of likelihood, and if there is any chance at all of evading them or blunting them, that chance will be heightened if we know what the dangers may be and consider in advance how to prevent or ameliorate them.
Staring at danger may not be pleasant-but closing your eyes will not make the danger go away, and with closed eyes you will surely be destroyed by it.
Isaac Asimov
Part 1. Universe Destroyed
In A Choice of Catastrophes, "Catastrophes of the First Class" are those in which the whole Universe is destroyed.
Actually, the possibility of such a catastrophe long antedates the imaginings of modern science fiction. In ancient times, it was usually taken for granted that the Universe would be destroyed someday (as it was created) by the Word of God, or by the decree of Fate.
Even today there are many who assume that there will be a Day of Judgment and that it is even imminent. In every generation there are those who await it momentarily ("The Last Trump" by Isaac Asimov). And, of course, the end can come about through the action not of the Creator of Humanity, but of the Created of Humanity ("No Other Gods" by Edward Wellen).
If we put mythology to one side and confine ourselves to the even mightier and more colorful conclusions of science, we do not have the crash of the Lord as He slams shut the Book of Life, but rather the long, long dwindle of sound ever, ever fainter as the Universe whispers dyingly to its death; as entropy increases, ever more slowly, to its maximum; as available energy dwindles to zero and with it all change, life, us ("The Wine Has Been Left Open Too Long and the Memory Has Gone Flat" by Harlan Ellison).