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 most relevant case is, if there is a Patientenverfügung, that demands no life support is performed. The moment 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 a similar fashion, in the absence of such a document, 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 but Behandlungsabbruch (end of a medical procedure), which is defined as the not providing or ending of procedures that could prolong life. It 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 alife is made clear, either by the patient, his Patientenverfügung or verified by the next of kin.
Because Mallory is not committing any crime in that case, 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.