What is the theme in Loitering with Intent by Muriel Spark?

Another theme in this book is strength of character. "Well, what I found common to the members of Sir Quentin's remaining group was their weakness of character. To my mind this is no more to be despised than is physical weakness. We are not all born heroes and athletes. At the same time it is elementary wisdom always to fear weaknesses, including one's own; the reaction of the weak, when touched off, can be horrible and sudden."