For individuals, that is simply wrong. If my brother were to kill an innocent kid and that kid's father were to kill my brother, as much as I love my brother, I'd hate him for killing an innocent kid and I'd totally understand the father of the kid killing my brother.
Would I suffer? Sure. And I'd be ashamed of what my brother did. But I wouldn't go out and kill the kid's father.
It's totally ridiculous to consider killers doing mass shooting should be given a second chance to mass shoot again and it's totally normal parents of some of the dead kids want lifetime jail (or the death penalty).
And I don't think the parents of the mass shooter would, in turn, go on a killing spree. They know fully well what their kid got is well deserved.