The right measure of the penalty
On the axis: The meaning of punishment · the penalty
Retributivism measures the penalty by the gravity of the wrong (eye for an eye), deterrence by what it takes to discourage: two measures that can demand far more, or far less, than each other.
Justification
Punishment is demanded by justice itself, as retribution proportionate to the crime (juridical law of retaliation), never as a mere means of social utility: punishing someone to deter others would treat him as a means. This is classical retributivism.
Metaphysics of Morals, Doctrine of Right, §49 E
Justification
Punishment is in itself an evil, a pain, and is justified only by the utility it yields: all punishment in itself is evil; upon the principle of utility, if it ought at all to be admitted, it ought only to be admitted in as far as it promises to exclude some greater evil. Its first end is the prevention of offences, chiefly by deterrence and secondarily by the reform of the offender; Bentham explicitly rejects retribution and the lex talionis as an evil added to an evil, with no profit.
All punishment in itself is evil. Upon the principle of utility, if it ought at all to be admitted, it ought only to be admitted in as far as it promises to exclude some greater evil.An Introduction to the Principles of Morals and Legislation, ch. XIII, §2
An Introduction to the Principles of Morals and Legislation, ch. XIII-XIV (punishment, its ends and its measure)