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 Mel Gibson and Danny Glover's policemen characters waving a diplomatic passport around saying "Diplomatic Immunity!".
At which point, Glover's character 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 just been revoked".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 have been likely consequences - for them as police officers as well as for United States - if this has happened in real life?
I would especially prefer answers that reference specific actual precedents of a similar occurrence, or established formal rules.