As a system insider, I'll say this...
I believe that the death penalty is misapplied in our system. A guy who robs a store for drug money and shoots/kills a clerk may be rehabilitated. The death penalty in this situation is about revenge, not necessarily justice. Yet this is a very common application of the death penalty, and often a situation where the case is later reversed due to new evidence. Life in prison is a better option.
Take a guy who is a serial child rapist, and I have great doubts that you will ever be able to fix that, no matter what meds or therapy you use. Apply the death penalty to this guy. If nothing else, you'l never have to worry about this one molesting a child again, so it's protection of society. Yet I know of no jurisdiction that even allows the death penalty in these kinds of cases. In fact, at least in Ohio, absent an application of physical force to compell the molestation, a defendant isn't even eligible for a life sentence. And, if a judge applies a maximum sentence in these cases, there is an automatic appeal of the sentence.
As for the OP, if the new law does not state that it is to apply retroactively, it shouldn't be applied retroactively.