Why would Iran take out after Saudi Arabia? Among other things, Saudi Arabia is a majority Sunni nation; Iran is the modern home of the Shi'ite branch of Islam.
One of the nations which will benefit from this whole episode is little Bahrain sitting in the Persian Gulf.
The majority of the Bahraini population is Shi'ite, but the royal family and much of the industrial class is Sunni.
This incident gives credence to Bahrain's claim that, during the "Arab Spring" uprisings, the unrest was being orchestrated, and paid for, by Iran.
This is an extremely dangerous time for the Obama Administration and for the Western world in general. Israel and the Palestinians are warily watching each other to see if each is serious about re-opening peace talks even while the military junta in Egypt appears to be losing control over the traditional secular nature of that country.
If Ahmadinejad feels that he is being boxed in, he will double-down on his influence in Syria and Lebanon and try to deflect attention to this plot by urging his client states to cause new trouble in Israel.
As to domestic politics, if it begins to sink in that Iran is so dismissive of Obama that they would hatch a plot to kill the Saudi Ambassador by blowing him up in a restaurant in the middle of Washington, DC this does not help the President's re-election campaign.
Those pesky Persians.