The first Russian soldier to be tried for war crimes pleaded guilty to killing a Ukrainian civilian on Wednesday as fears grew over the fate of hundreds of Ukrainian soldiers who surrendered at the Azovstal steelworks in Mariupol a day earlier. Ukrainian officials said they are negotiating to exchange the beleaguered fighters for Russian prisoners of war. Russia’s parliament was expected to but did not take up a resolution Wednesday blocking the swap. Lawmakers had cited the Azov regiment, a militia with neo-Nazi roots that was absorbed into Ukraine’s military and which Moscow says still comprises Nazis. Troops from the...