While I understand where you are coming from, I think you may be missing the reasoning behind why many of us are offended by this shirt. Most of us aren't offended because "he hates America", but because someone made a shirt depicting a violent act in which thousands of people died. You very well may be correct in your first two bullet points, which is why what really bothers me is the fact that this shirt even exists in the first place. I think it's sad that ANYONE would support or mock the death of innocent civilians in any place in the world. I am just as offended when I hear Americans joking about starving children in Africa, tsunami and earth quake victims, Holocaust victims, civilian deaths in Iraq and Afghanistan, etc. I know many people will tell me to lighten up, but as someone who is involved with global health and health care I think one of the worst things we can do is dehumanize other people/cultures in order to "lighten up". Unfortunately, though, we all do it. Not just Americans, but everyone, because it's easy to do when you are raised your entire life in a particular culture and taught to believe certain things.

This brings up your third point. I think that a lot of Americans are fully aware that we aren't the "good guys"; particularly Americans on Reddit. All you have to do is ignore the popular media and actually pay attention to the occupy movement as well as approach people in cities and college campuses to find this sentiment.

Also, while your fourth point may be correct, it also may be false. It actually makes me think of the Confederate flag. Many people fly the flag as a symbol of Southern pride, but ultimately to a lot of people it signifies a time period and culture of inequality and hatred. Therefore, even though the people flying it MAY not be flying it to represent hatred and white pride, because others construe it as hateful it is still offensive and morally shouldn't be flown in public places (note I said morally, not legally. Legally you should be able to wear/say/fly whatever you want no matter how offensive). While that is not the best metaphor, it still explains why this shirt is offensive even if the man wearing it is actually depicting the loss of what he considers to be American hegemony as opposed to killing Americans.

I just don't see how being offended by something like this makes one an ignorant American patriot. People were murdered, and the t-shirt is depicting this horrific event. That is offensive. It's offensive when it happens anywhere, and to celebrate the death of civilians (despite whether you agree with their country's political agenda or culture) is extremely sad to me. This guy may not know what this shirt means, but clearly the person who made it did. Therefore, I don't see how this SHIRT is not offensive.

Edit: TL;DR Many of us are offended because the shirt depicts a murder scene in which thousands of innocent civilians were killed. I am just as offended when I similar memes/shirts/jokes about similar incidents that occurred in other countries (even if the US was to blame).