I will repeat for HROT my views on why the death penalty is wrong in no particular order.
1. The OP's story is used as a justification for why the death penalty is needed, but crimes this heinous are not the ones where the death penalty is most applied. And, the more heinous the crime, the more bias in the jury becomes a factor. People have issues thinking objectively, especially when kids are involved.
2. Statistics show the death penalty is applied most to poor people, black and brown people, and most often to poor black and brown people. The Fugitive was a neat movie, but right white dudes don't get sentenced to death. Juries, cops, prosecutors, judges... They can all be biased.
3. Many prosecutors have political ambitions, and they push death penalty cases in order to burnish their credentials. Governors refuse to step in and use their powers to offer commutations or pardons. Think of Bill Clinton when he was running for POTUS and Arkansas executed an imbecile. Clinton could have, and should have given a commutation, but he knew he'd be attacked as soft on crime.
4. Death penalty cases are expensive. If society pushes for the death penalty, they need to fund an appropriate defense. Especially to fund expert testimony or scientific tests.
5. The death penalty has proven to be highly stressful on the corrections staff who carry out the sentence.
I'm leaving off a few things, but, this is just off the top of my head.