Recently, he was finally brought to book. His conviction was a vindication of the rights of the Guatemalan people and a victory for human rights workers. Until he was unconvicted. A Guatemalan court ruled the conviction invalid.
Why? The ruling business elite of Guatemala thought indicting and convicting Rios Mont was a bad precedent. Committing genocide against the people, especially the Mayan speaking Indios, who are at the bottom of the social pecking order, is a time-honored tradition of Guatemala's ruling elite. If Rios Mont is guilty of genocide and punished for it, why stop with just him? A lot of rich and connected people are also complicit.
Of course, the US government backed up Rios Mont's tantrum of mass killing as part of saving the free world from godless communism (it was the 1980s, terrorism hadn't moved up to the top of the list yet). Another significant player in the massacres was the Israeli army. They trained the Guatemalan military's death squads and gave them inspiration. This is also well documented, if any one cares to look it up (don't ask Abe Foxman).