Acton on Rhadamanthus?
I used that quotation in the Epilogue to Emancipating Slaves, Enslaving Free Men, with the following footnote:"I have been unable to find the source for this quotation attributed to John Emerich Edward Dalbert-Acton, 1st Baron, so it may be apocryphal. But if Acton did not actually say it, he should have. It is consistent with his thought and his other writings." I now have found a source that makes it highly unlikely that those were Acton's exact words but still leaves it somewhat uncertain whether he at some point made a similar reference to Rhadamanthus.
I initially found the quotation in an article by Murray Rothbard, who provided no supporting reference. Acton had said something related in his well-known correspondence with Bishop Mandell Creighton, which is also the source of Acton's oft-repeated maxim about power corrupting. Acton wrote with respect to the crimes of great men,"I would hang them higher than Haman, for reasons of quite obvious justice, still more, still higher, for the sake of historical science," but there is no reference to Rhadamanthus.
Only recently was I looking through the 1907 edition of Acton's Lectures on Modern History, edited by John Neville Figgis and Reginald Vere Laurence. Here is what the two editors write in their introduction about"Lord Acton as Professor" on p. iv:"For to Acton history was the master of political wisdom, not a pursuit but a passion, not a mere instrument but a holy calling, not Clio so much as Rhadamanthus, the avenger of innocent blood." So the reference to Rhadamanthus comes from them, although it is still possible that they are reporting on something Acton had previously said.