How would you react if you were on a flight where a family was boo'd for being allowed off the plane first to receive their soldier's body?

I am not sure, and hope I never find out, but I suspect that at best I would be banned from travel on that airline for a VERY long time,

Probably get in trouble.

Stand up & salute them until they have departed the aircraft...then return the salute slowly & deliberately.

They should be punished. Guess they forget the price freedom An because of the Military is why they live the way they do

Entitled anal by-products such as these know little of shame or responsibility - they are blind to the idea of sacrifice and duty. In an emergency, I would gladly hand them a fake parachute.

I would tell them to go live in a third world country. There are too many American citizens that have become "Entitled" to their citizenship and don't know the sacrifices soldier & their families make.
Here is an excellent example...I was horrified for the parents when I learned this in history about the Sullivan Brothers.
https://www.history.navy.mil/browse-by-topic/disasters-and-phenomena/the-sullivan-brothers-and-the-assignment-of-family-members0.html

In truth, I most likely would most likely ask, the Rude & Insensitive people , to please show some Respect for the Dead, and for the Grieving family, who lost a loved one, and are currently Grieving
& going though a hard time.

Then I would make every effort not to become violent.
I would do this by speaking to the airline personal and military Justice overview, for victims of hate crimes , I would request military Legal personal look into this matter to see if they can put pressure on the airlines to look into there rule book, to see if they can Legally Justify removing the rude, & Insensitive people off the Flight, & ask the airline to to see if they can legally place the rude & Insensitive people on the airlines no fly list. thus allowing the airlines to refuse service to these people.

simply state "when you're picking up you Dead Family Members body, then you can get priority debarkation too"

As f*ck'd up as so many people are nowadays, I seriously doubt that would ever happen. Don't try to convince me otherwise.

I would walk up to that individual and slap them in the face or spit in their face. I would have no qualms about doing that at all.