There are so many home grown anti- government militia groups that have done a lot of damage. One example is the militia group that recently took over Federal land because they wanted to use the land for their own purpose, such as grazing. There are also the lone wolf killers that are bombing abortion clinics and shooting up the workplace. Timothy Mcvey was one of the most notorious ant-government killers that set off a huge bomb at a Federal building, killing men, women and children. There are so many killers right here at home that they make the Islam killers look like babies.
Terrorism from Islamic groups is mainly what the media shows and wants to show. It is more sensational and feeds the anti-muslim sentiment that already exists in the United States. Just because we do not see other terrorist groups does not mean they do not exist, and, if anything, they are more dangerous because we do not see them.
The hatred expressed by so-called Christians around the world leads to unnecessary attacks and killing of people based on pure ignorance. The people who use Islam for their own ends cannot be excused for their terroristic acts, but as with history, the vast majority of damage is caused by Christians who take the holy higher ground.
Islamic extremists are simply more prevalent at the moment and more adept and using the foibles of the modern world to their advantage then any other group. The problem is proportional to the base from which it draws, which is to say there are simply more people who follow the teachings of Allah than who follow the preachings of an angry Northern Irish priest.