I think the issue on tastelessness is that it has to be sincere, as if the person making it really thought this was a good idea, and it also helps if it is funny. Look at any market of film and you will see Comedy is allowed to be as tasteless as it wants to be, but drama, not so much. I think if the guy who made the School Shooter Mod had something of a sense of humor (even if it was only funny to him), the shock wouldn't have been near as bad, but this guy made this with an intention of being absolutely serious.
Look up (actually, don't) any of those videogames made by White Supremacists (I don't have a name in front of me), they believe their agenda is so appropriate, that putting what they want in their games is a good idea (Like advertising about "Realistic Sounds" and putting the sound of a chimp screaming in place of a human scream), and it is more poor taste than an actual good idea. Mortal Kombat, on the other hand, has this really over the top sense of humor about itself and it is the reason why it was so big (and is coming out with Kratos as a fighter on the PS3 this year).
Big Difference in genre, but it all makes sense when you think about it. Even if Postal II was in bad taste, it meant to have a sense of humor about itself (Though nothing about it was funny) and it works for some people.