I didn't really understand the half-a-bullet part, but the crux of the joke to me was that he wanted to kill himself before it could turn into an ending like "The Mist".
In case you have not seen "The Mist" here is a big spoiler. Don't read if you want to see the movie.
The Mist is about a mist that rolls into a town, and with it horrible monsters. A bunch of people hide out in a grocery trying to survive the creatures, but most in the end get killed, with the main group (including a father and son) escaping in a car.
At the very end of the movie, the car runs out of gas in the middle of the mist. Rather the get eaten by the creatures, they decide to all commit suicide. The father kills all the other survivors, including his son, by shooting them in the head. When it comes to his turn to end his life, he finds himself out of ammo, and survives.
In a horrible twist, he gets out of the car in time to see a huge group of military coming through the mist and burning it away, dozens and dozens of trucks of survivors being moved out of the region. If he has waited five minutes longer, none of the others would have had to die.
The joke to me was that he killed everyone, and was about to kill himself with the fear that it would end up just like the ending of The Mist. He shoots himself, but survives, just in time to hear the military say "Zombies are gone, everything is great, have a nice day!" which was part of the reason he was even trying to kill himself.
It's not the best joke, and I hated having to explain it, but it gave me a chuckle.
Basically what Steinman said: the joke of this is that it's a ripoff/parody of The Mist. But like a CAD comic, you basically give away the punchline two panels ahead of time, make a blatant reference to said movie, and then basically do the EXACT same ending.
I don't see where this was supposed to be funny, clever or original.