You’re arguing the merits (or rather the reverse) of the death penalty which is a non-issue to me. I think that God knows the heart like he does the hairs on your head, and time is not what is needed for redemption. I tend to think it’s more cruel to keep a person locked up for the rest of his life, which we do to murderers but not innocent babies, to further point out that the death penalty and abortion are not equal arguments. But as I said, it’s a non-issue to me. I see no justice or injustice in it.
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.