Although the Church is not entirely opposed to the death penalty, it has spoken against it. So have many liberals. Is this one issue in which the Church and liberals see eye to eye, or are there different reasons for their opposition?
One thing to keep in mind is that the Church has not always been against the death penalty. For example, St Augustine:
However, there are some exceptions made by the divine authority to its own law, that men may not be put to death. These exceptions are of two kinds, being justified either by a general law, or by a special commission granted for a time to some individual. And in this latter case, he to whom authority is delegated, and who is but the sword in the hand of him who uses it, is not himself responsible for the death he deals.
St Thomas Aquinas:
Now every part is directed to the whole, as imperfect to perfect, wherefore every part is naturally for the sake of the whole. For this reason we observe that if the health of the whole body demands the excision of a member, through its being decayed or infectious to the other members, it will be both praiseworthy and advantageous to have it cut away. Now every individual person is compared to the whole community, as part to whole. Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good, since “a little leaven corrupteth the whole lump” (1 Corinthians 5:6).
Vengeance is lawful and virtuous so far as it tends to the prevention of evil. Now some who are not influenced by motive of virtue are prevented from committing sin, through fear of losing those things which they love more than those they obtain by sinning, else fear would be no restraint to sin. Consequently vengeance for sin should be taken by depriving a man of what he loves most. Now the things which man loves most are life, bodily safety, his own freedom, and external goods such as riches, his country and his good name. Wherefore, according to Augustine’s reckoning (De Civ. Dei xxi), “Tully writes that the laws recognize eight kinds of punishment”: namely, “death,” whereby man is deprived of life; “stripes,” “retaliation,” or the loss of eye for eye, whereby man forfeits his bodily safety; “slavery,” and “imprisonment,” whereby he is deprived of freedom; “exile” whereby he is banished from his country; “fines,” whereby he is mulcted in his riches; “ignominy,” whereby he loses his good name.
On the other hand, he also says:
Two vices are opposed to vengeance: one by way of excess, namely, the sin of cruelty or brutality, which exceeds the measure in punishing: while the other is a vice by way of deficiency and consists in being remiss in punishing, wherefore it is written (Proverbs 13:24): “He that spareth the rod hateth his son.” But the virtue of vengeance consists in observing the due measure of vengeance with regard to all the circumstances.
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The Church’s argument has become more opposed to capital punishment over time because it is seen, in this day and time, as not being necessary anymore for society to defend itself against the criminal and that retribution could be as effectively gained (thus providing for commutative justice) by depriving an individual of his liberty for life.
I don’t know the liberal thought process on it (other than I’m certain that it is somehow “feelings” based), but that, to my understanding, is how the Church has evolved over time.