Lady Of The Lake
The druid girl glanced up at the moon as the midnight bells rang
The girl was turning. This is what Gaius had talked about. The curse turning this druid girl into a bloodthirsty beast.
He drew his sword.
And then she screamed, a painful, agonizing, sorrowful, scream.
And in that moment, he realized. He realized that she didn't want to kill. It wasn't her choice. It hurt for her as she unwillingly morphed into this creature.
She was so small, so fragile looking, but he had no choice. He had to kill her, before she killed again, next time it being someone he cared about like Morgana, Gwen, or, even Merlin.
I'm sorry, he thought to himself as he struck the final blow, and she disappeared, presumably to die somewhere peaceful, little did Arthur know that it would be Merlin who was holding her as she died.