Natalia Chorna had warned her more outspoken twin sister to be careful after Russian forces occupied their home town of Skadovsk near Kherson, southern Ukraine, in February. But Tetyana Mudryenko found it hard to keep her anger about the war to herself. Last month, Mudryenko paid the ultimate penalty for proclaiming Skadovsk Ukrainian territory. According to several witnesses, she was dragged into the street by the self-appointed pro-Moscow authorities and hanged in a public execution. “In occupied Skadovsk, you can’t have your own opinion,” said Chorna, 56. As Ukraine pursues its counteroffensive in Kherson and Russia forcibly relocates tens of...