> This is why a horrible crime like rape is typically not punished as strictly as murder: if the penalty were the same, why not do away with the witness.

As an aside, something feels off about this example. Choosing to differ the penalties to incentivize not murdering the victim suggests that the murder outcome is less desirable, i.e. the crime is more horrible. The argument implicitly assumes that murder is the more horrible crime while making a case for why the punishments differ. Given that punishments are generally expected to be in proportion to the horribleness of the crime that assumption suggests that the punishments would differ to begin with.