So, I started playing Postal and it seems that the world is literally out to get the protagonist. Like all the hostiles are already in place - armed police, SWAT, militia with explosives, etc.
That's pretty extreme. So, it's more of a zombie apocalypse game than a mass shooting game?

Here's a simple summary of the plot which is kept deliberately vague in the game:

"From what can be gathered through the original website, manual, and the game itself, an average man known only as the Postal Dude discovers that a mind altering substance has been released on the town of Paradise, infecting the populace with insanity and bloodlust. Capped off by his sudden eviction from his home, the Postal Dude believes himself to be the only sane man left and sets out to put an end to the madness in his town, which he believes originates from the local Air-Force Base."

For some reason I always thought it's a serious game. Reading the mission descriptions in manual and stuff yesterday suddenly made everything click together. It's one big joke.

What's really out where? Mass murder is ultra-rare per capita. Murder is already pretty rare and mass murder is much much rarer - especially one that isn't gang related or isn't just perpetrator killing himself and his family.

Also, I think that treating the subject as a joke is much more "problematic" than making a serious, realistic game about the subject. War is arguably a much worse subject and people have no problems playing/making serious war games. I think ultra-serious grimdark Carmageddon would work quite well.

Basically, you a homicidal nut who thinks that everybody but himself is insane and out to get him.
IMHO the problem is the game's dark comedy is not nearly as clever or witty as the designers thought it was.
Playing as a homidical maniac is fun for a bout ten minutes, but then it gets tired, particularly since your foes are very repetitive and are pretty stupid.