User blog comment:MerlinUSA/Lies My Dragon Told Me/@comment-5577678-20130129184358/@comment-5995315-20130130152319

Technically, he told Merlin that Morgana was now the darkness to his light, which would imply that it was potentially avoidable.

That's part of Merlin's dilemma. If he had killed Morgana as soon as the dragon said "The witch must die" he would have lost his moral agency, and if he had killed the child Mordred on the basis of a prophecy he would have lost his humanity altogether.

What he did try to do was contain both situations as best he could. To me that was the humane thing to do, even if didn't work. I don't think the dragon understood Merlin's hesitation because he could not read Merlin's mind.