I would say that the line which implies the author thinks Helen's behavior is going to hurt Helen herself is Don't cut your fingers with the edge / Of your keen wit
Because her mind and behavior are going to hurt her, or rather make her hurt herself.
It is foreshadowed that Susan is at a zenith, a highpoint in her life. This, however seems to be the last highpoint that she will ever be at, insinuating that the rest of her life was much more depressing and less exciting.
I think it it a fragment by think I mean I think I know the sentence
