Considering the group which has taken the responsibility for this attack[1], is a militia organization, I'd say that 2nd amendment is already being exercised by a minority which feels that they are being targeted systematically by the same group of people who are sworn to protect them.

The only difference is that they don't tout second amendment, but founding fathers envisioned precisely these scenarios when they advocated for 2A.

> Besides the advantage of being armed, which the Americans possess over the people of almost every other nation, the existence of subordinate governments, to which the people are attached, and by which the militia officers are appointed, forms a barrier against the enterprises of ambition, more insurmountable than any which a simple government of any form can admit of. Notwithstanding the military establishments in the several kingdoms of Europe, which are carried as far as the public resources will bear, the governments are afraid to trust the people with arms. And it is not certain, that with this aid alone they would not be able to shake off their yokes.< - James Madison, Federalist Papers 46[2]

1. http://www.mirror.co.uk/news/world-news/dallas-police-shooti...

2. http://www.constitution.org/fed/federa46.htm