Brenton knew what had happened. He saw the gills on my neck when he pulled me out of the water. Being that she did not have a tail yet, meant that the full transformation had not completed. There was only one way of stopping the spell. He would have to kill the merman who tried to change his wife. If he doesn't she will not survive the night.
Brenton was not a serpent. He was a land dragon. It was luck that he came up on the lake when he did. If the foul beast had mated with his wife again, she would have become a full mermaid. The beast had been chasing her too.
Brenton did feel some sympathy towards the beast. He did save his wife's life. That is the only reason he does not just have sushi. But if he is forced to chase this beast, especially in the water, he will be having a fish fry.