My professor said something similar but I disagreed. I thought it really holds the novel together. St. John is a controlling and abusive dick kind of like how Mr. Rochester was in the previous volume but he absolutely is not willing to give Jane any credit. At least Rochester somewhat respected Jane. So he makes Rochester look better in hindsight because of how controlling he is but he cannot be criticised because he is a good Christian right? I disagree he goes off and dies in India while everyone else live rich and fulfilling lives.

Look at how Rochester changes completely after his 'fall'. Rochester is capable of both respecting Jane and changing to not being controlling while St. John can only see one way. He is similar to Rochester but in the end he is a foil that redeems Rochester by being so terrible.