This girl’s name is Ace. She has the power to destroy reality. Ace was dying, and the Justice League and CADMUS believed that before she died, she would take reality with her.
Obviously heroes like Superman could never kill a child. He could never do what needed what needed to be done. But know who could?
Batman was charged with the task of killing Ace. Batman made it to where she was held up, and instead of killing her, he sat beside her on a swing set. Not caring whether the world was destroyed or not, Batman sat beside her until she died, so she wouldn’t be alone.
This is probably one of the best scenes from the DCAU, it’s from the JLU episode, “Epilogue.” I can’t recommend it enough. Go watch it. Now.
Also, later on Batman named his dog after the child.