. >> reporter: in spite of the murder charges, sandy is out on around in 2017, prosecutor colleen barnett has a challenge. proving that sandy ended decades of apparently happy marriage with that kitchen knife. and barnett admits, she can't say why. >> in texas we don't have to prove a motive. i thought that i was going to be able to prove that she did what she did, that i wouldn't be able to prove motive. >> that raised red flags for me. this woman doesn't seem like she could be a killer, but the prosecutor says she is the killer, but we don't know why. >> reporter: barnett says a possible motive came to her in the middle of the trial. >> and it wasn't until the jehovah's witnesses testified from the defense and i learned a little bit about the religion that i thought possibly that might have been part of it. >> reporter: defense witnesses say the melgars appeared to have had a good marriage. nevertheless, barnett conjures a theory. sandy wants a divorce, but, afraid of being shunned by her fellow jehovah's witnesses, rd >> don'ts. at a unown.e i'll nev waityou hearn band.orteut if th bar