I was watching some documentary on various types of extinction level events that can destroy life on Earth (e.g. say a major asteroid strike etc). One of the more interesting ones was a gamma-ray burst from some collapsing star within our galaxy. As it collapses, two bursts come out of opposite ends, and any thing in its path that is an organic lifeform is done (apparently even the most radiation resistant organism on Earth, some bacteria, would not be able to escape the effects). According to the program, one of the largest mass extinctions on Earth (there have been 5 of them ...some interesting reading there) called the Ordovician Extinction that killed the second largest number of lifeforms of the 5 mass extinctions was probably caused by a gamma-ray burst.
The good news? Most people exposed to it would go quickly. It is only those not directly affected who would die slowly.
Which is why I always say that one shouldn't worry too much, because if something bad ever happens it will probably be like the end of the Sopranos ...the screen will simply go black.