CAIRO — Gathering Thursday morning around a mosque used as a morgue for hundreds killed the day before, many Islamists waited confidently for a surge of sympathetic support from the broader public. But it failed to materialize. With their leaders jailed or silent, Islamists reeled in shock at the worst mass killing in Egypt’s modern history. By Thursday night, health officials had counted 638 dead and nearly 4,000 injured, but the final toll was expected to rise further. A tense quiet settled over Cairo as the city braced for new protests by the supporters of ousted President Mohamed Morsi, after...