It absolutely is a stretch. You're comparing apples & oranges. To me it seems like saying "A soldier kills militants in XYZ country. Therefore it's not a stretch to say he kills his family if he's mad."
I don't find this analogy meta-discussions valuable. The only stretch is whether or not the killing could happen inside USA. Of course it (perhaps) hasn't happened yet, but that's what we are discussing: if it could happen. Any evidence on the truth of "could it happen?" has to has some differences, otherwise it would have already happened and we wouldn't be debating this in the first place.
EDIT: One intelligence agency, CIA, has already killed an American citizen[2].
1. http://www.guardian.co.uk/commentisfree/2013/feb/05/obama-ki... 2. http://www.washingtonpost.com/wp-dyn/content/article/2010/01...