In the book, there were no outright murders in the back stories. The servant didn't smother the old lady, he withheld her medicine so that she died "naturally"
If this could have been proved this would be a crime, possibly even murder. There is intent leading directly to the death. An accused might have pled guilty to manslaughter/culpable homicide.
The detective didn't stamp on the man in the cell, he fabricated evidence and the hapless victim died shortly afterwards in prison. No intent to kill and no direct link to the death.
The General didn't shoot his love rival in the head, but deliberately sent him on a dangerous mission into enemy fire. Again no intent and no direct link.
The point of the book is that they weren't murderers in the eyes of the law, but amoral/reprehensible people who had got away without being punished for causing these deaths
Marston had no intent. His actions might have been reckless or careless driving but causing a death by driving doesn't necessarily mean the more serious crime is committed.
Armstrong might have been tried for manslaughter /culpable homicide.
Brent- no crime at all.
Claythorne- clearly intent - I wonder if that would be another manslaughter /culpable homicide.
Lombard - no crime, no duty of care owed to the natives and no intent.