I think from the reading 'the Turn of the Screw' it is most likely that the Governess has some kind of mental disorder, most likely paranoid schizophrenia. Where the subject experiences paranoid delusions accompanied by Hallucinations
The images of the ghosts could be these hallucinations and the distrust of the children fits well with this illness, she most probably was told stories about Quint and Miss Jessel by the other maids and became convinced of their existence, this idea builds up in her mind, untill she kills Miles in some kind of fit of paranoia that he is in league with these 'Ghosts'
here is a brief description of the illness
Paranoid schizophrenia is the most common type of schizophrenia in most parts of the world. The clinical picture is dominated by relatively stable, often paranoid, delusions, usually accompanied by hallucinations, particularly of the auditory variety, and perceptual disturbances. Disturbances of affect, volition, and speech, and catatonic symptoms, are not prominent.
Examples of the most common paranoid symptoms are:
* delusions of persecution, reference, exalted birth, special mission, bodily change, or jealousy;
* hallucinatory voices that threaten the patient or give commands, or auditory hallucinations without verbal form, such as whistling, humming, or laughing;
* hallucinations of smell or taste, or of sexual or other bodily sensations; visual hallucinations may occur but are rarely predominant.
Thought disorder may be obvious in acute states, but if so it does not prevent the typical delusions or hallulcinations from being described clearly. Affect is usually less blunted than in other varieties of schizophrenia, but a minor degree of incongruity is common, as are mood disturbances such as irritability, sudden anger, fearfulness, and suspicion. "Negative" symptoms such as blunting of affect and impaired volition are often present but do not dominate the clinical picture.
The course of paranoid schizophrenia may be episodic, with partial or complete remissions, or chronic. In chronic cases, the florid symptoms persist over years and it is difficult to distinguish discrete episodes. The onset tends to be later than in the hebephrenic and catatonic forms.
The general criteria for a diagnosis of schizophrenia (see introduction to F20 above) must be satisfied. In addition, hallucinations and/or delusions must be prominent, and disturbances of affect, volition and speech, and catatonic symptoms must be relatively inconspicuous. The hallucinations will usually be of the kind described in (b) and (c) above. Delusions can be of almost any kind of delusions of control, influence, or passivity, and persecutory beliefs of various kinds are the most characteristic.