Steve,,depression and schizophrenia are pretty well the same territory of mental illness in general. It's such a miserable existence anyway,,,and so is vital that all medication is taken regularly and the patient monitored. Nearly all psychotic drugs are subject to blood-testing periodically,,and I can only assume that Sheilas' missed appointments were because she was taking other substances which counteracted her medication and that's why her condition worsened.
The deteriorating circumstances of her condition rendered her to uncontrollable psychotic outbursts,,,so it's more than feasible that she carried out the killings.