[bobthechef]

The important thing to notice is that he author isn't talking about retribution being wrong per se, but private vendettas. Justice is by its very nature retributive. You cannot show mercy if retribution is not the deserved and proper response to a crime. One difference the author alludes to between justice meted out by the state and private vengeance is that the latter is characteristic of blood feuds.

[linkregister]

Yet the most effective justice systems focus on public safety and rehabilitation. See Norway, et. al.

Though I agree whole heartedly with your conclusion, that blood feuds never have a settled score, while an individual’s debt is (theoretically) paid when the State has meted out its punishment.