Letting something not get to us =/= making fun of what happened. You can mourn the dead, respect those left, yet still get on with your life, all without needlessly doing douchebag things like posting that .gif. I'm not angry at you, I just enjoy seeing someone as stupid as you trying to defend your fucked up position on this topic. Should you manage to actually come up with some legit justification, then I WOULD accept your opinion on the matter. You, however, have not. Quite the contrary, you have contradicted yourself. You originally said that you could do it on this site because of the anonymity associated with it. Then, you said that you should do it because it would stick it to the terrorists who tried to be nasty to us by attacking us. However, due to the aforementioned anonymity, there would be no way for them to know that it was an American posting the .gif, or any western citizen for that matter, and not in fact one of the terrorist's compatriots. As such, both of your arguments are mutually exclusive, and thus invalidate each other.
RT has already set the standard by not allowing any sort of pornographic images on their site, even when externally cited and produced. From that, it's a mere extrapolation to determine what parts of the internet we do and don't allow on here. It takes very little effort to understand that which only /b/ will find humor in, and to know that those topics should be kept to /b/, or at the very least, the /b/ enclaves of other sites; for example, the BAR in the Basement. Granted, the BAR is hardly what it used to be, but the same is true for /b/ as well.
In short, a little common sense will go a long way, should you desire to not piss people off in a thread where the whole intent is to post funny, interesting, or original .gifs, and the intent certainly ISN'T to make people cringe when remembering the worst terrorist attack on American soil. And should you desire to hide behind your "It's the internet, it's anonymous!" facade, which is a weak one at best, then that's certainly your right, Mr. Screen Name, just like it's my right to tell you to fuck off when you do.
No, it won't be funny in a couple months. Or a couple years. Or ever. Fuck you for finding humor in the senseless slaughter of over 3000 people, including the people who ran INTO the buildings to try to save innocent lives after the planes hit before the towers actually fell. You insensitive fucking prick, eat shit and die.