If I were trying to parse this out based on what little information we have, but with some little knowledge of terrorist history, I would rank the likelihood of who perpetrated the act thusly:

A. Some Islamist terrorist (single or group, perhaps even home-grown) wanting to strike at he U.S., and the marathon presented a broad target, hard to fully protect.
B. A non-Islamist person or group with a specific gripe against government(s) or perceived powerful groups with which (t)he(y) disagree, and has chosen the site as above.
C. Someone from group A who relishes sticking it both to the U.S. And Israel, and took advantage the chronological confluence of the marathon and Israeli independence day.
D. A newly hired ER doctor, looking for a lot of quick experience. This sounds far fetched, but a local fireman, a few years ago, turned arsonist so he could become a hero smoke eater.
E. Somebody with no affiliation wanting to murder a relative, spouse or someone with whom they hold a real hate for, with an opportunity to make it look like one of the groups above was involved. "Law and Order" come to life.

I'm sure GregF. and Keith and others can add more categories. Heck, we can even start a pool to see who's right!

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My real suggestion would be for us to grieve for the victims, belay the mindless speculation. Let the investigators do their job until they find answers, and can bring culprits to justice.