Many states in the US explicitly say you can resist an officer using force when they shouldn't. Many juries, DAs, and other cops in said states won't give a shit.
As for the going against the constitution, the word murder in the executive order basically removes any contradiction, because murder is a legal term in the various penal codes and wouldn't fit someone who had shot a cop lawfully.
It'll get shuffled into some lower charge than murder. I remember a story where one cop drove past another during a traffic stop, doing something in the order of 100+ mph, hit the other one, dragged him a distance, and yup, the dude who got hit and dragged died. He wasn't charged with murder, but a lower manslaughter. The shit-for-brains actually pled not guilty, rejecting a plea bargain.
The reason he drove so fast by the other? It was a prank that supposedly was sometimes done.