It might work for you, but are you telling me that if you were the murderer in question, you wouldn't be afraid that the family wouldn't assassinate you at first opportunity if this happened to their daughter?
A heavy sentence is safety for the criminal as well.
That seems completely orthogonal to the point I disagreed with, which was (loosely, sorry) that punishment purely for punishment's sake is a social good.
A heavy sentence is safety for the criminal as well.