This turned out to be SOJOURNERS Magazine (January 2013 issue), a magazine I'd never heard of by a group I'd never heard of. Turns out to be a religion-based social justice group arising out of the anti-war protests of the Sixties: Xian but no aligned w. any specific denomination, focused these days on pacifism and ministering to the poor.
The article in question, "Gandalf, Gollum, and the Death Penalty" (p. 22-27), by Tobias Winwright, takes Gandalf's famous comments to Frodo about not being too swift to deal out judgment, esp. re. putting others to death. Winwright freely admits to being no Tolkien scholar and not knowing whether JRRT himself opposed or supported the death penalty, yet finds these words and this exchange a good starting point for a discussion of why he opposes the death penalty.** Here's the piece:
The one deliberate execution I can think of offhand in Tolkien's work is that of Eol the dark elf, thrown to his death by order of Turgon king of Gondolin for killing the king's own sister, Eol's estranged wife. (SILM.138). And Tolkien is careful to emphasize Eol's malevolence (using a poisoned weapon and keeping that fact secret till it was too late to neutralize the venom).
There are plenty of deaths in battle, with no quarter asked or given,*** but deliberate murder is quite rare in Tolkien (e.g., Smeagol's strangling of Deagol, Wormtongue's killing and eating Frodo's cousin Lotho) and tends to have a deeply corrosive effect on the killer or to show that they are utterly depraved (the Feanoreans who leave Dior's children to starve).
In the end, I'd say this piece shows less that Tolkien opposed the death penalty than that he had a nuanced approach to this, as so many things.
Also, while on the topic of Inklings-related pieces spotted in the library's mazagine section that day, there was also XIANITY TODAY (December 2012 issue), whose cover story was "There and Back Again", about near-heaven experiences (but on the cover only; inside the story itself had a more literal an un-Tolkien-themed title, which I forget). This same issue did turn out to have a story about C. S. Lewis, entitled "Why MERE XIANITY SHOULD HAVE BOMBED", by Jn G. Stackhouse (p. 38-41), which I didn't have time to read (or, frankly, inclination, not rating MERE XIANITY among Lewis's better books). Here's the link:****
--In addition to this article, I see they've done several about Tolkien recently (see http://www.christianitytoday.com/ct/topics/t/j-r-r-tolkien/
for a list) and have even put together their own e-book, available on Kindle et al:
Unfortunately, so far as I can tell this isn't available in book form, just electronically.
current reading: THE PHOENIX AND THE MIRROR by Avram Davidson 
GREEN SUNS AND FAERIE by Verlyn Flieger 
*about the work of Edward S. Curtis, one of the most interesting men I'd never heard of but will now definitely be wanting to know more about
**(as do I)
***(Tolkien was no pacifist, but almost all his good guys are fighting defensive wars, not launching wars of aggression)
****(having now gone back and done so, I think the one line that'll stay w. me is Stackhouse's description of CSL as "the Ulstercum-Oxonian equivalent of a 'good ol' boy with a Ph.D'."