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?

It is pointless to tell an arrogant fool they need to respect the families loss. Then calmly shove an elbow into there mouth.

I will be sure the booer(s) are last off the plane.

First thing, to Show the Family Respect and let them Debark the aircraft. Then to explain to those who may not understand what is taking place and ask them to be respectful to the Family for there loss and respect the Service Member who lost there life.

Be Pissed at the assholes

No point in saying anything! Show respect with a salute and catch the boo’er in the latrine after we get off the plane.

I think I would be getting a two week leave after letting said individual feel the pain of the whole family. ( some where private like escort him to the bath room so he does not get his sickness all over the Plane).

Say something like my son made it possible for you to live and work in a free country. So you can afford first class.

Ideally, a blanket party would be in order but given the fact they these assholes have a monopoly on classlessness, I would embarrass the shit out of them by educating them in front of the other passengers after the family respectfully disembarks the plane. Make them feel an inch tall.

How would I react?

Poorly.

I'd be inclined to work with the aircrew to hold the booing passenger onboard until ALL of the other passengers were off the plane.