“if laws were incredibly hard on those that commit serious crimes, you mark my words, crimes and murders would plummet,”: as proved during the Middle Ages when Crimes rates was so low. Oops, no.
You have to know one or two things: Punishment is not a deterrent, and never was.
Two main reasons:
I planned to commit a crime, so I planned to fool the police and to escape from the punishment. Nobody knowing he will be caught commit a crime or offence. If you over speed it is because you think there are no speed camera at this spot, or no police officer.
A crime committed under influence, hunger or other strong feelings are not under hard penalties.

To make things simple:
To kill your wife and her lover you find in bed is not a cold blood murder.You would kill then without thinking of the consequences, so hard punishment prospects didn’t stop you in doing it.
If you plan to kill my wife and her lover later on, you will produce/imagine a plan to avoid to be caught. And you think you will be better than the Police.