GAZA CITY, Gaza—More than a month after The Daily Beast reported evidence suggesting Israeli soldiers carried out the summary execution of six men amid fierce combat in late July, there are no signs that the Israeli government is investigating the matter. It has declined to respond to repeated inquiries. The independent organizations Human Rights Watch and Amnesty International, which the Israeli authorities have barred from Gaza, are unable to follow up in detail. And a United Nations investigation of abuses on both sides is barely under way.The Daily Beast has continued its own investigation, however. And the picture that’s emerging...