My theory is still that an episode of Rizzoli & Isles inspired this.

True, although the international element of the Olympics makes that one a little bit easier to figure, I think.  (Granted, always much easier long after the fact when the killer has already revealed his motive.)  That one would've suggested some kind of militiaman kind of crazy (New World Order, UN, global communism/lizard people, etc).

Marathons are more city things, and if a Rudolph type were going to bomb a marathon (at least one in the states), surely NYC would be the one to expect it to occur at?

Be nice to America. Or we'll bring democracy to your country.