Another factor to consider in the question of why someone would leave casings at the scene of a shooting is that, even though some serious advances were being made in the scientific study of ballistics and forensics at that time, they weren't widely known. Few criminals would be concerned about leaving empty casings at the scene compared to leaving one or more corpses, bullets and all. If police and attorneys weren't certain of the probative value of empties, why would you expect a criminal to be aware and take precautions.
I knew burglars 25-30 years ago who didn't start wearing gloves until after they had been sent up once on the basis of fingerprint evidence. And they had grown up watching Dragnet.
A final thought, and it speaks more to the question of coldbloodedness, is that certain infamous killers over the years have been known to leave "calling cards" recognizable within the community and intended to "send a message". A pile of .38 cases or a stone under the victims head - just a matter of taste.