Emily Soriano was doing laundry this week when a friend rushed into her house with news about their yearslong quest for justice.

In December 2016, gunmen stormed into a house in their poor neighborhood north of Manila and started shooting. They killed seven people, including three children and a pregnant woman. Ms. Soriano and her friend, Isabelita Espinosa, each lost a son, both teenagers.

To the victims’ families, the massacre seemed senseless, like thousands of other extrajudicial killings carried out during former President Rodrigo Duterte’s so-called war on drugs. Ms. Soriano and Ms. Espinosa have long held him responsible for the deaths of their sons —Angelito Soriano, 15, and Sonny Espinosa,16 — whom they say were innocent.

Mr. Duterte’s arrest on Tuesday, on accusations of crimes against humanity, was a major milestone toward accountability, the women said.

“What matters to me now is that justice is served,” Ms. Espinosa said. “And that we don’t allow these butchers and tyrants to remain in power. We must fight back.”