How can this possibly be a positive benefit in her elderly years?

I’m not a parent, but I’ve seen something in parents that lose a child (that grew up enough to have a personality and experiences). They are forever injured. Its something they never heal from. The hurt never goes away from the loss. I can only imagine the child being murdered would amplify this.

I am not a parent, which means I’ve never been in a position to lose a child to murder, but your question does come across with, I believe, an unintended arrogance, to tell an elderly parent that it “[isn’t] a positive benefit” to be focusing on the murderer of her child.