Kylo was good with a broadsword. Arthur and the other knights had completely fallen for Merlin's story of finding a man in the forest who was like Lancelot had been; a commoner with skill with the sword who had managed to train as a knight. It had taken some persuading for Kylo to agree with the disguise, but Merlin had won in the end.
Now, as he watched the man subtly knock Arthur's feet from under him with magic and land him in a large puddle, Merlin knew that his instincts to show him some mercy had been correct. There was an aura of darkness hanging around him, but beneath it was a ribbon of light.
Plus, it was awesome to watch Arthur floundering in a mud puddle.