There's an old trial attorney maxim when you've got a witness on the stand: "never ask a question if you don't already know the answer." In this week's murder trial of Patrick Frazee for the death of his fiancée Kelsey Berreth, prosecutor Jennifer Viehman followed that rule with every question of Frazee's secret girlfriend Krystal Lee, ending with one final, heartbreaking response.

Lee provided bombshell testimony, telling jurors how she'd been at home in Idaho when Frazee called to summon her to Woodland Park, Colorado, because she had "a mess to clean up."

Lee, who says she was both in love with and fearful of Frazee, drove nearly 12 hours, picking up the keys to Berreth's home from Frazee and performing a massive cleanup of blood and evidence there.

Lee says Frazee came clean to her about what happened – he'd gone to Berreth's home concealing a baseball bat on Thanksgiving Day 2018. With their 1-year-old daughter Kaylee playing in the next room, he blindfolded Berreth, pretending he was going to light scented candles and play a guessing game with her to see if she could name their fragrances. Instead, he pulled out the bat and struck Berreth an estimated 10 to 15 times.

But it was what Lee told the jury when the very last question was posed to her that stunned the courtroom and proved why that old legal maxim exists. As Lee sat on the witness stand, Viehman asked if Frazee had told her what Berreth's last words had been. Lee paused and began to cry. She said Frazee told her that as he beat Berreth to death, Berreth had uttered two final words – "please stop."

From left, Teller County, Colorado, prosecutors Jennifer Viehman and Beth Reed, and DA Dan May CBS News

At that moment, everyone in the courtroom sat stunned. Afterward, District Attorney Dan May told "48 Hours," "just saying it in that manner told you so much about Kelsey and what a wonderful person she was -- to even just say the word 'please' while he's beating her to death. The jury needed to hear that. They needed to hear how brutal this was, how uncaring he was. How senseless this was."

Frazee did not heed Berreth's final plea for her life, but on November 18, 2019, a Colorado jury stopped him a different way – finding him guilty of murder and sentencing him to life in prison.