How was killing the sisters the final nail in the coffin to Trujillo's regime?

After "Trujillo was gone," the trial of the murderers was on TV for a month, and they admitted to killing the Mirabal sisters and Rufino by strangulation. Then they put them back in the Jeep and pushed it over the edge of a cliff at 7:30 at night. This part of the story was reported to Dede by Mateo Nunez, who traveled all the way from his "remote mountain shack" just to tell her what he knew about her sisters' death. Although the murderers were sentenced to long terms, "all of them were set free during our spell of revolutions." Manolo, Leandro, and Pedrito are all released, and Trujillo is assassinated.