I
finally found it, after far too much searching and finally spending over an hour reading a most entertaining account of Abraham Lincoln's days as a lawyer in Illinois because it seemed to fit the search parameters. I now know why the question's topic is familiar!
Source:
https://www.knox.edu/documents/pdfs/LincolnStudies/Burlingame, Vol 1, Chap 9.pdf
Michael Burlingame –
Abraham Lincoln: A Life – Vol. 1, Chapter 9, p. 994
I am
Abraham Lincoln (certainly a key figure in the Lincoln administration!). From the above account:
"One of the more controversial murder cases was that of Isaac Wyant, who attacked his neighbor Ason Rusk with a knife. In self-defense, Rusk shot his assailant in the arm, which was subsequently amputated. Thirsting for revenge, Wyant in 1855 shot Rusk to death in cold blood. Lincoln helped prosecute the case. . . .Leonard Swett, who defended Wyant with the novel insanity plea, won despite Lincoln’s best efforts. Later, when informed by authorities at the state insane asylum that Wyant was truly deranged, Lincoln expressed regret for having prosecuted him so vigorously."
There's been too much about Dirty Dan Sickles lately, which is why I almost was led astray, especially because searching the insanity plea inevitably came up either with Dirty Dan's case, or jumped straight from M'naughton to John Hinkley. I finally decided to search on the last part of the question, and it worked!