Is it more sinful to kill somebody who is saved, or somebody who is not?

Neither. Sin does not require a victim. It is an act an individual commits. It's the same with secular law. When a person commits a crime the offended party is the State because it is the State whose command has been transgressed. We oftentimes mix and mingle the victim in and out of the process, but it is the State that brings the charges against a person.