I think Lightningrunner made a rash decision in the heat of the moment. She clearly hadn't planned to hurt Estherfern when they all went out that day, but emotions that had been brewing for sometime finally boiled over. And those emotions are valid, even if what they led Lightningrunner to do was not.
I do not believe that Lightning should have to spend her eternity wandering the dark forest, never to see her family again, for such a mistake. Especially when no cat was hurt.
This isn't to say that her crime should go unpunished, but it's my belief that Lightningrunner already paid that price: with her life. One in which she had many more years to potentially live as she was so young. In death, she'll have more than enough time to reflect on what her anger cost her.
I LOVE how everyone has a different perspective on this.