TB, I interpret the scene between Frankie and Tom (one of the best in the trilogy, IMO) as a negotiation. Frankie had to be eliminated for the same reason that Fredo had to be killed. Forgiveness is not possible in that world. Frankie traded his permanent silence (which could only be guaranteed by his death) for his family's security, both physical and financial. Frankie makes the offer and Tom accepts the deal.