Doctor assisted suicide - a hot potato in the western countries these days. Although this is a kind of suicide, the patient is fully awake and living human being in contrast to a patient in life support due to brain death. According to the Buddhist teachings, what are the karmic repercussions for the doctor who assists the suicide?
Assisting suicide would be considered an offence of defeat in a monk.
Whatever monk should intentionally deprive a human being of life, or should look about to be his knife-bringer, he is also one who is defeated and is no more in communion.
First among the five precepts, acknowledged by all Buddhist practitioners, is not to take a life, especially a human life. Consequently, from the Buddhist point of view, that the time of death is uncertain precluding the taking of life to be determinable, both suicide and the one who assists suicide generate extreme negative Karma. In the case of euthanasia, the individual concerned is already actively dying, and the outcome is certain. It can be argued that "mercy killing" in the last hours is an act of compassion where the Karma is neutral; i.e., the patient will die if I act...the patient will die if I do not act.
Finally, I would point out that your example of the patient who is brain dead, by definition cannot make a decision for Physician assisted suicide; and one cannot determine the number of years (with an otherwise healthy body) that that individual would live. So to take that individual's life would be a clear case of murder. It is the karma of such a person describe in your example to be and remain in that state until karma determines their life span.
The Pali scriptures explain 'kamma is intention' (AN 6.63). Thus the results of kamma (an action) will follow the quality of intention. The moral precepts are training rules thus not 100% fixed for lay people (however, the killing precept is 100% fixed for monks, since many of the monks rules are about the public image). That said, it is not considered bad karma for a mind free from greed, hatred & delusion to commit suicide. Therefore, a person that commits suicide due to unbearable physically debilitating (rather than emotional) pain is blameless. As for the doctor or helper...?
[Channa said]...Friend Sāriputta, I am not getting well, I am not comfortable. My painful feelings are increasing, not subsiding;…their increase and not their subsiding is apparent. I shall use this knife, friend Sāriputta; I have no desire to live...
[The Buddha said]...Sāriputta, when one lays down a 'body' ['kaya': grouping of five aggregates] & takes up a new 'body', then I say one is blameworthy. This did not happen in the case of the bhikkhu Channa; the bhikkhu Channa used the knife blamelessly.
Suicide is one of the greatest of sins that one could do unto self. In the Vinaya generally, suicide is condemned. Assisting or encouraging suicide is equal to the gravest offences. A monk who deliberately ends the life of a patient, even from compassionate motives, is expelled from the monkhood and can never re-ordain in this life, so there’s no room for assisted suicide in Supreme Buddha’s dispensation. It is said that this act will take you to the worst hell - the great hell / Ānatareika Niraya. Now you know the danger of this Samsara. You and I would have committed suicide a countless number of times in our previous births. This shows the urgency and importance of becoming a Sotha-Aapanna – to get into the stream – the Noble Eight-fold Path.