Was Augustine opposed to the death penalty? In Henry Chadwick's translation of the Confessions, I find him making a surprising claim:


  Like most of the Church Fathers, Augustine was against capital punishment. (page 21)


The context is Augustine's critique of the orator who uses his skill of persuasion without regard for the outcome:


  A man enjoying a reputation for eloquence takes his positions before a human judge with a crowd of men standing round and attacks his opponent with ferocious animosity.  He is extremely vigilant in precautions against some error in language, but is indifferent to the possibility that the emotional force of his mind may bring about a man's execution.


I see nothing there indicating that Augustine is opposed to capital punishment.  And the City of God, Book 1, Chapter 21, suggests the opposite position: 


  The same divine law which forbids the killing of a human being allows certain exceptions, as when God authorizes killing by a general law or when He gives an explicit commission to an individual for a limited time. Since the agent of authority is but a sword in the hand, and is not responsible for the killing, it is in no way contrary to the commandment, “Thou shalt not kill” to wage war at God’s bidding, or for the representatives of the State’s authority to put criminals to death, according to law or the rule of rational justice. [emphasis added]


Henry Chadwick has forgotten more about Augustine than I will ever know, so surely there is some basis for his claim that Augustine was against capital punishment.  So:


Do other writings of Augustine indicate an opposition to capital punishment?    
Did his views change over the course of his life?  
Did he accept capital punishment in theory, but oppose it in practice?
Henry Chadwick translates the relevant passage:


  A man enjoying a reputation for eloquence takes his position before a human judge with a crowd of men standing round and attacks his opponent with ferocious animosity. He is extremely vigilant in precautions against some error in language, but is indifferent to the possibility that the emotional force of his mind may bring about a man's execution.36


Here, Henry gives the clear impression that Augustine's illustration is alluding to the possibility of a real execution.

However, J.G. Pilkington renders the passage this way:


  When a man seeking for the reputation of eloquence stands before a human judge while a thronging multitude surrounds him, inveighs against his enemy with the most fierce hatred, he takes most vigilant heed that his tongue slips not into grammatical error, but takes no heed lest through the fury of his spirit he cut off a man from his fellow-men.


And Edward Bouverie Pusey renders it:


  In quest of the fame of eloquence, a man standing before a human judge, surrounded by a human throng, declaiming against his enemy with fiercest hatred, will take heed most watchfully, lest, by an error of the tongue, he murder the word "human being"; but takes no heed, lest, through the fury of his spirit, he murder the real human being.


Both of these translators are giving us Augustine's portrayal of a man whose unbridled anger destroys his brother in the same sense Jesus explains in Matthew 5:21-22

Conclusion

Based on Henry Chadwick's likely misunderstanding of an important detail in Augustine's illustration, together with the lack of citation for his claim in the footnote, I wouldn't put a great deal of faith in his having accurately remembered Augustine's position on capital punishment. Perhaps he is inadvertently giving us his own.