How is pride linked to destructive magic? How is selflessness linked to creative magic?

How does Charn serve as a warning to Polly and Digory about humankind?

How does Digory’s decision to ring the bell affect his life? How does it affect the lives of the people around him and those of people he hasn’t even met? Why might Lewis have chosen to introduce evil to Narnia in this way (i.e., from the outside rather than from within)?