In chapter 5 of the novel Frankenstein, by Mary Shelley, we find Victor reacting to the "birth" of his creature. Contrary to what he thought it would be, the process would be painful and horrible for Victor.
He describes the moment when the creature opened its eye. Victor foolishly thought that it would be a moment not too different than that of the creation of man, basically, with all things going perfectly well.
However, he saw a horrid creature that appealed to Victor's emotions. Victor was disgusted by this and his first reaction was to run away from the creature. In the process of conceptualizing what he did, he thought about the fact that he had let a monster lose; What would it do? How bad will it all get?
Victor suffered what amounted to a nervous breakdown and Clerval, his dearest friend, took care of him. The birth of the monster is what creates the anxiety amounting to agony. That, and the fact that Victor is now solely responsible for the fates of those who come across the creature.