Depends on motive.
Probably no religion is immune to having adherents that are not true adherents or who might flip out and turn murderous. Or this guy could be completely irreligious
However, Islamists may very well feel that they are honoring the many Islamist mullah's calls for death to Americans. Hence my question.
As it has been frequently said on these boards. All Muslims are not terrorists, but all terrorists are Muslim. While not an absolute truth by no means, it is hard to point to Christian terrorists. Timothy McVeah of the OKC bombing is frequently pointed to, but there is substantial evidence that he was working with Iraqis.
The answer behind this atrocity is yet to be known.