For years, the fiery band of activists pushing for democracy in China looked to Liu Xiaobo, the jailed Nobel Peace laureate, as a source of inspiration. They created social media groups devoted to his iconoclastic poetry. They held up his photos at rallies, demanding justice and transparency. But Mr. Liu’s death last week of liver cancer, after a final, futile attempt by friends to bring about his release, has dealt a withering blow to the pro-democracy movement. Some say it is now at its weakest point since the Tiananmen Square crackdown in 1989.