In my opinion, what causes the creature to change is the way that the people in the village treat him.

The creature has learned pretty much everything he knows by watching the people.  He has not realized that they will be horrified by him.  He goes and talks to the old man, who accepts him because the man is blind and cannot see his face.

But then when the other people come back and see him, they are repulsed and they chase him away.  It is right after this that the creature meets and kills William Frankenstein.