And that's bad because...? We can't succumb to lawlessness if people don't respect the law and expect no penalties. If they did the same thing at Area51 and were shot, would you say it's the fault of the guard who swore to protect their post just as the National Guard does? Is the value of civility not greater than a single life?
The 2nd amendment was intended to keep people with a healthy fear of one another. The equation is clear - to take another's property means one must risk something even more valuable; one must risk their life. It's that inequality that preserves order and to try and rebalance the equation by cancelling out the ever-present constants and insist life is above both liberty and prosperity, then you are fundamentally altering the social construct in untenable ways.
So then why don't we just have the police shoot on sight for any suspected crime? Police get called to store theft, just shoot the suspect. Police get called to vandalism, shoot first.
I don't follow the logic that a crime against property should become a capital crime just because it is down by a large(unorganised) group of people. Furthermore, the constitution guarantees a fair right to a jury trial, no matter how egregious, and police or the national guard shooting to kill looters goes against that.
Should drug smugglers be shot too? They are also going against societal order.
A rebellion is different, if you have to deploy the military because people have overpowered the police then it is no longer just a normal crime scene. Normally people would just disperse when that happens, but if they tried to attack the military in the same way they attack the police then what would you expect?