Suppose you believe that final death is an appropriate punishment for someone's crime. You also believe cryonics has probability p of working, where p is too small for you to sign up for cryonics. Should you allow the prisoner to be frozen?

If cryonics doesn't work, it doesn't make a difference; and if cryonics does, you shouldn't. Thus there is no reason to allow the prisoner to sign up for cryonics.

Notice that this conclusion doesn't depend on the probability of cryonics working, or on how certain you are the prisoner is guilty, for any certainty sufficient to justify him being executed.