Yesterday Bryan Caplan asked:
What’s wrong with revenge? To be more specific: Suppose X is the most severe morally acceptable punishment for act Y committed by person Z. Suppose that the government fails to do anything about Y. What’s wrong if a person personally affected by act Y does X to Z?
In the comments, Matt C looks to bias:
[Compared to public revenge,] personal revenge is too likely to get the wrong guy, and personal revenge is too likely to be excessive.
Acad Ronin looks to feuds:
"[The rule of law is] the transfer of the duty of revenge to the Queen. It’s the officers of the Crown avenging a man’s murder, not the man’s father or the family. Without law what you have is feud, tangling between themselves, and murder repaying murder down the generations. As we have here. But if the Queen’s Officers can be relied on to take revenge for a killing, then the feuding must stop because if you feud against the Queen, it’s high treason. That’s all. That’s all that happens in a law-abiding country: the dead man’s family know that the Crown will carry their feud for them. Without it you have bloody chaos."
Chisolm, P.F. 1994. A Famine of Horses. New York: Walker and Co.
If the feud problem itself results from biases, these could be the same explanation.
When someone kills another, there are disagreements - disagreements about the justification of the act, disagreements about the nature of the punishement, disagreements about how to deter this act in the future (even assuming all the facts are known).
In revenge, the opinion of one person dictatorially provides the answer to these questions, without an elected govenment or a market to balance other people's opinion. In everything else, we've learnt to compromise, to balance different people's wants and priorities. Why should justice be different?
"If the feud problem itself results from biases, these could be the same explanation."
Instead of bias, disagreement. We often end up with disagreement on just what 'X' is, owing to variation in morals, and we sometimes disagree on the identity of Z. The natural thing for people to do when they disagree vehemently on important matters is to feud, and the seriousness of the feud is a function of the seriousness of disagreement. We delegate to government the power to resolve serious conflicts to protect ourselves against very costly reprisals, continuing ad infinitum, which come with a host of externalities and probably don't satisfy the most interested parties anyway.