There are several parallel stories told in the documentary “Finding Oscar.” The main one lays out, with traditional means, the horrific circumstances of a 1982 atrocity perpetrated by Guatemalan army commandos against the residents of a small Guatemalan village. Taking its name from the location of the raid, the so-called Dos Erres massacre left 250 civilians dead — many of them unceremoniously dumped down a well, with some thrown in while still alive — by a squad of elite military operatives, known as Kaibiles. The Kaibiles had gone in search of guerrillas they suspected of having recently ambushed government soldiers during the country’s long-running civil war with rural leftist rebels.