Sorry for my English, I am not a native speaker.
This episode shows one of the Merlin's traits that I love most: despite being the hero of the show, he is able to kill or let die the villain in order to save Arthur or Camelot. He was very close to do it seasons ago, when he threw a distracted Morgana through the stairs, although in the very last moment he healed her. Now, he has stuck to his decision, no matter how tough it was.
That said, I think he is making a huge mistake (which is great because perfect characters are unappealing and mistakes are the driving force of the plot). His assumption that Mordred is evil is going to lead Mordred to evil, as it happened with Morgana. Morgana was desperate to find someone who accepted and loved her as she was. Merlin could (and should) have been that person (I mean love in a friendly way, although a romantic relationship would have been awesome too). I am not saying that Merlin is responsible for Morgana's change. Of course, Morgana is the ultimate responsible for her own actions, but Merlin did not help.
On another note, in this episode we see the darkest side of the old religion. In other episodes, druids, warlocks just want Arthur to establish religious freedom in Camelot. Here, the Triple Goddess gives the king an ultimatum to submit himself to her authority, otherwise she will doom Camelot. I hope there are other (and better) gods, but that point of the old religion is unclear to me.
I am eager to know what happens next!! I love this show and each of its characters and the acting is amazing.