In Lethal Weapon 2, the bad guy — if memory serves, a diplomat from South Africa — commits a bunch of crimes. Then, as he's getting away on a yacht, he taunts the policemen characters played by Mel Gibson and Danny Glover, waving a diplomatic passport around and saying "Diplomatic Immunity!". At which point, the character played by Glover promptly shoots him in the head, using a clearly legal response of "... just been revoked".
Now, this being a Hollywood film, they rode off into the
susnset sequel; but what would the likely consequences have been, both for them as police officers as well as for United States, if this had happened in real life?
I would especially prefer answers that reference specific actual precedents of a similar occurrence, or established formal rules.