On top of the staircase near the entrance to the downtown Los Angeles courthouse where Erik and Lyle Menendez were convicted nearly three decades ago of killing their parents in a salvo of shotgun blasts, a woman trumpeted a sign that read, “35 years time served. Let them out!”

Dianne Martin first learned about the case from Ryan Murphy’s polarizing nine-part series detailing — and dramatizing — the 1989 killings of José and Kitty Menendez. That piqued her interest, which led her to watch a Netflix documentary released earlier this month on the brothers. She said she prefers the documentary, though she credits the series for spotlighting the case.

Monsters: The Lyle and Erik Menendez Story “blurs the line,” she explained. “You don’t know what’s true or not. I saw the documentary, and then I realized they had such a troubled life at home.”

Martin was referring to newly uncovered evidence...