What if it really comes to pass that Trump orders civilians killed?

So, let's say it actually happens. There is a large protest in Washington DC. Trump is in the White House enraged and calls Hegseth telling him to teach the protestors a lesson. Hegseth has spent the last several months purging the military and he knows that he has an Army Division Commander in his pocket who has formed various military units from MAGA minded soldiers, many of whom were facing charges, but these were dropped under the condition that they followed orders without ever asking questions.

The next thing we know, an Army unit is in DC and open fires directly into a crowd of protesters. Fifty dead right off the bat. Artillery is then fired from a nearby staging area and several more killed. By the end of the day, over five hundred dead, some of whom were shot down in the street while already wounded. Trump says they deserved it and says anyone who asks awkward questions will get the same. Hegseth says he will follow any order his Leader gives, including killing unruly and protesting civilians, and that includes members of Congress.

What would actually happen if this took place?