1) It seems rather lacking of an actual argument. It looks like he makes a big unsupported leap somewhere in (4) and (5). He seems to just define his way to his conclusion that retributive punishment is legitimate.

2) There’s also, arguably, an excessive mixing of law and morality here. The talk of blame and forgiveness evoke morality more than law, and the impulse to punish out of revenge, over and above ending and rectifying threats/rights-violations (as far as possible), stems from that. This may be part of the leap I pointed out above: how does one get to the legal “it is right/legitimate to punish” from the moral “this person is blameworthy for committing a wrongful act”?

3) Restitution, ostracism, and the like can serve as “punishment” even while that is not their primary purpose. One can accept that oneself and others are liable for these things if we or they commit rights violations, particularly serious ones, without seeing some non-functionalist, non-instrumental (particularly vengeful) retributive punishment as necessary for mature, moral, flourishing human beings and communities.