It was really almost on a different wavelength than the viewers watching. As a youngster watching I thought the two cops (one played by later superstar Morgan Freeman) just pulled up in a squad car and then just shot Christopher Plummer's character dead. Actually Plummer is in a hands in the air position with the cops but he then sees the very beautiful Sigourney Weaver coming and he is so ashamed of what he has done (and for getting caught)--he will never be able to face the very, very great woman again! So therefore he pretends he is reaching for a gun and only then the squad car cops shoot him dead.

(Think about it. What if my first interpretation had been correct. Would have invited endless speculation. "The cops had an APB on this man that he was such a fast and crack shot that he could pull a gun seemingly out of nowhere and shoot you dead. So the cops simply had to shoot him first without him a chance to be taken alive!".)