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 think passive aggression is best. As I passed their seats, I might ( accidentally) fall into them and fuck them up as much as possible. Tray tables, drinks Etc. Then appoligise and hope they would take a swing at me.

I have a difficult time believing this actually happened.

I would drag his ass down the stairs and make him face the fallen soldier's family and apologize. Of course he would obviously press charges for assault in which case I would make it a much more obvious assault charge...!

Very sad conduct. In contrast, when my husband returned for Vietnam, his flight into LAX was delayed an hour. By the time he landed, everyone knew my situation and made sure I was closest to the jetway. The flight attendants allowed he to exit the plane first (probably 1st class). Nobody stopped us from running to meet in the jetway.

I would get up and use some good Marine Core Language, if that was not enought, this old man can still dance

One at a time , or all at once.

no way could i sit there and not say anything

Honor is demanded

Pick one of the "booers" out and stare (what my daughter would call a "death stare"). Staring can make someone real self-conscious about something they are doing that is repulsive.

I would consider if they’re flying first class in a pandemic and it’s not for business they’re selfish assholes.

Never been in that situation, but I'd probably get up and LOUDLY reiterate WHY they're allowed to leave the aircraft first!