What to Watch Friday: What’s the fatal flaw in this ‘black widow’ murder scheme?

Here’s what to watch tonight.

20/20: Fatal Flaw - On the Rocks (8 p.m., ABC)

The second installment of this new four-part ABC series looks at a case of a so-called “black widow” killer who framed her own daughter for the deaths of her two husbands. Stacey Castor of Weedsport, NY, was convicted in 2009 of killing her second husband, David Castor, for attempting to kill her daughter, Ashley Wallace, after framing Ashley for his death. Stacey poisoned David with antifreeze in 2005 and attempted to kill Ashley in 2007 with a mixture of pills mixed into vodka, orange juice and Sprite. After all this, the death of her first husband, Michael Wallace, also fell under suspicion.

Lifetime made a movie based on the case called “Poisoned Love: The Stacey Castor” story.

So what was the fatal flaw in Stacey Castor’s plan to get away with murder?

That’s what we learn tonight. This program uses firsthand accounts from journalists and law enforcement officials to recount the twists and turns of criminal investigations, focusing on “breakthrough moments that led investigators and police to each killer’s doorstep.”

There’s commentary from ABC News contributors Elizabeth Vargas, Ryan Smith and Matt Murphy. Episodes stream the next day on Hulu.

The first episode of the series looked at the case of Raven Abaroa, who went on trial in Durham for the stabbing death of his wife, Janet.

Dateline: The Diane Kyne murder (10 p.m., NBC)

When Diane Kyne was found dead in her Seminole, Florida, home in August 2010, competing stories emerged about which of the two most important men in her life may have killed her. The 49-year-old woman was found strangled in her bedroom and the only other people there at the time were her husband, William Edward Kyne, and his 23-year-old stepson Kevin Kyne. Each man accused the other of being responsible for the murder. Dennis Murphy reports.

Some programming descriptions are provided by networks.