"Or he/she was a pro and left the casings and a chunk of the gun on purpose to cast doubt on the idea that he/she was a pro." I think a more likely explanation

"Or he/she was a pro and left the casings and a chunk of the gun on purpose to cast doubt on the idea that he/she was a pro." I think a more likely explanation would be that the killer was a pro, and didn't care about the brass since they planned on dumping the weapon, which couldn't be traced to them anyway.