It is very important to realise that the majority of critics argue that this book is not about a battle between the forces of good and evil at all. Rather it is a very complex psychological study of one young woman and the way that she is overpowered by the character of her employer, and thus "creates" in her own head a situation which she believes is a battle between the evil of Peter Quint and the former governess and herself for the souls of the children.
There are a number of proofs that such critics use to argue this position. Firstly, and most importantly, they look to the way the employer is described compared to the governess:
This person proved... this prospective patron proved a gentleman, a bachelor in the prime of life, such a figure as had never risen, save in a dream or an old novel, before a flustered, anxious girl out of a Hampshire vicarage... He was handsome and bold and pleasant, offhand and gay and kind. He struck her, inevitably, as gallant and splendid, but what took her most of all and gave her the courage she afterward showed was that he put the thing to her as a kind of favour, an obligation he should gratefully incur.
The way in which he obviously charms the governess and has a great impression on her young, naive and rather innocent character is obvious. Likewise, critics point to the fact that it is the governess alone who sees the apparitions and argue that, in the knuckle-whitening finale, the governess actually suffocates Miles in her embrace as she seeks to protect him from the advances of Peter Quint. Therefore it is important to be aware that this novel is more about psychology and the unreliability of the narrator than it is about good vs. evil.