It’s a very good question.

Only God can truly forgive sins, as only He has perfect justice and knows every heart.

And only God can judge repentance.

Hence every man alive should fear the final judgment ("I never knew you; depart from me" — may we never hear those words)

Human forgiveness is to be humble to God. We are to “forgive” even unrepentant sinners from our heart (exactly as Erika did) as it removes our judgement, cleanses our heart by the grace of God (to not hold it “on our ledger” as she worded it).

Otherwise, us holding back forgiveness waiting for repentance is to judge repentance ourselves, making ourselves into God.

fwiw, by this same humility, the civil authorities are required to enact worldly justice by the authority God has granted them. This means even if the killer repents very publicly and very sincerely, he is still to receive the punishment due (likely capital punishment) — and again for the same reasons that only God can truly judge repentance, and only God can truly forgive us our sins.

The thief on the cross is instructive here. He is granted forgiveness before his death, which by his own admission/confession he rightfully deserved.