Time is needed for repentance, and repentance is needed for redemption. Yes, this is an argument against the death penalty in general, provided that the guilty party can be incarcerated securely.
Forgiveness is individual; some might find it in their hearts to forgive without repentance, others won't. We should strive for it, but that has nothing to do with the operation of civil justice.
Well, there is a little bit of time between the final sentence and the cold needle (way too much time by some opinions). If a person can’t repent in that time, they might never. I see what you mean, however.