They summoned a whistleblower to Capitol Hill, but instead they got a virtuoso storyteller. Gregory Hicks, the No. 2 U.S. diplomat in Libya the night Ambassador Chris Stevens and three other Americans were killed, was to be the star witness for Rep. Darrell Issa (R-Calif.), the man leading the probe of the Obama administration’s handling of the attack on the diplomatic outpost in Benghazi. But despite Issa’s incautious promise that the hearing’s revelations would be “damaging” to Hillary Rodham Clinton, Hicks didn’t lay a glove on the former secretary of state Wednesday. Rather, he held lawmakers from both parties rapt...