It depends a tad in germany...
Mallory is staff ordered to shut off Life Support
There are cases where life support can be turned off. The two most relevant case are the patient being away telling the doctor, or the patient being unconscious and if there is a Patientenverfügung, that demands no life support is performed. The moment the patient says so or the properly documented paper is in the hands of the doctors, they have to end all life-prolonging measures, and shut off life support - in fact, they are not even allowed to start such measures once they are informed of the wish for it. In a similar fashion, in the absence of such a document and with an unconscious patient, the next of kin might have the life support shut off at some point if they can make it clear that such treatment is against the patient's wishes.
In Germany, turning off the Life support is not (active) euthanasia (which is illegal) but Behandlungsabbruch (end of a medical procedure), which is defined as the not providing or ending of procedures that could prolong life. Behandlungsabbruch is not a crime, if demanded by the patient or the next of kin can make it clear that it would have been the wish of the patient. In fact, it becomes a crime to not turn off the machines the moment that the wish of the patient not to be kept alive is made clear, either by the patient, his Patientenverfügung, or verified by the next of kin.
Because Mallory is not committing any crime in case he is acting on the Patient's wish (expressed by the conscious patient, unconscious patient's Patientenverfügung or unconscious patient's next of kin in absence of a Patientenverfügung), Alice can't legally use any force against Mallory.
Mallory is just out there to murder
Simple case: yes, she may use a reasonable amount of force to stop Mallory.