Wolf –
No matter what a person does, we always have an obligation to save a neshama. Look in the gemara in Makkos (end of first perek) for examples of what lengths we would go to to try to save people who are accused of even the most horrible aveiros.
And especially considering that, as you said, we Halachically assume nowadays that anyone who r”l would do such a thing was not mentally competent, there is definitely an obligation to do everything possible to keep them alive.