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'd ask them to leave America and tell them another country is more deserving of their disrespect.

Depends on my audience.

Was there an announcement - did they know that's what this Family was doing?

I can't imagine anyone booing if they understand the situation. If they did,id follow the booers off the aircraft and give them hell all the way to the luggage area.

Just say "You're welcome you ----heads that service member died for your freedom to express yourself, as opposed to being executed on the spot for what you just did"

My actions at first would be a civil conversation they didn't act right after that well who knows let's hope I never am confronted with they issue

First off every one on the aircraft should have stood out of respect..but that aside, I would have been embarrassed but at the same time "really pissed off". I would have let my fellow passenger "in first-class" know what an incentive "ass-hole" they are.; to bad a fellow first-class passenger didn't have the guts to capture it on their cell phone and blast the "shit-heads" picture all over social media.
My prayers are with this family as they grieve for their fallen hero.

Personally, I would turn around and ask if any of them have served or lost a loved one. If they did then they should understand that THIS FAMILY JUST LOST A SON/DAUGHTER/ETC. If they have not had it happen to them, then they can kindly shut their traps and get over it.

I would pray for them that they would never have to experience the pain associated with the death of someone close to them who they loved and respected as much as the fallen veteran. There are a lot of people out there who have no passion or belief in something they would law down their life for and I feel sorry for them because they will never know what it really means to be alive. Jesus said in John 15:13 "Greater love hath no man than this, that a man lay down his life for his friends." Semper Fi

The same way I reacted right before I decided not to fight to stay in the service.
I came home for a few days because I had to go to court. Let alone with all the other BS going on in my life at that time I had this happen. I took my grandmothers car to the gas station with the 20 she gave me to fill her tank. With nothing more then my dog tags and clothes on. These outstanding citizens had their car hood up and trying to jump a car. I am not a racist. One man asked if I could help jump them. Sure I said. Hell I am a 63W it was what I did. Rescue broke vehicles from the field. After I determined it would not start he asked if he could get a ride up the road. No problem I said. After all I thought everyone thought as I did and there was no evil people here in the good ole USA. So it didnt click to me until we got up the road and I was led just a little further. At that point is when the gun was pulled on me. After a few minutes I told him he either needed to shoot me or get the hell out of the car. "I wouldnt say that today" + "Today I always have a gun ready"
After that day and a one sided review board set out to make themselves look good during Bill Clintons RIF I asked myself why in the fucking world would I risk my life for the likes of these fucking people.
So I will always be loyal to country, loyal to friends & family.
What I am saying is I would not even pay one mind to those fools. Because as you, the family of the fallen, are escorting the body of your soldier walk past them be proud. Proud that among the fools of this country, the court jesters, the pheasants your escorting a person who has given their life for the freedom we all enjoy.