Before Trump v US, if the President had somehow ordered all opposition murdered and the army agreed, exactly what would have happened differently? This is the point of the grumblings that I just don't understand. They imagine wild hypotheticals and erroneously believe that before Trump v US there would be some magic law enforcement agency stepping in to set things right.
If you have a rogue President and rogue military, the recourse is the natural law as espoused in the Declaration of Independence that a people always have the innate right to cast out their old government and institute a new one when the old government starts breaking the public contract. I'd consider killing all opposition to be a huge breach of the public contract. Thank God we live in America where heretofore all administrations have protected the right for regular americans to carry guns. Good luck to anyone who wants to actually try a coup!