Misunderstanding may arise by confusing the Buddhist and scientific definitions of death. Within the scientific system you spoke quite validly of the death of the brain and the death of heart. Different parts of the body can die separately. However, in the Buddhist system, the word death is not used in that way. You'd never speak of the death of a particular part of the body, but rather of the death of an entire person. When people say that a certain person died, we don't ask, "Well, which part died?"
Believe nothing on the faith of traditions, even though they have been held in honor for many generations and in diverse places. Do not believe a thing because many people speak of it. Do not believe on the faith of the sages of the past. Do not believe what you yourself have imagined, persuading yourself that a God inspires you. Believe nothing on the sole authority of your masters and priests. After examination, believe what you yourself have tested and found to be reasonable, and conform your conduct thereto.
There is, Oh Monks, a not-born, a not-become, a not-made, a not-compounded. Monks, if that unborn, not-become, not-made, not-compounded were not, there would be no escape from this here that is born, become, made and compounded.
Through countless births in the cycle of existence
I have run, not finding
although seeking the builder of this house;
and again and again I faced the suffering of new birth.
Oh housebuilder! Now you are seen.
You shall not build a house again for me.
All your beams are broken,
the ridgepole is shattered.
The mind has become freed from conditioning:
the end of craving has been reached.
As human beings we all want to be happy and free from misery.
We have learned that the key to happiness is inner peace.
The greatest obstacles to inner peace are disturbing emotions such as
anger and attachment, fear and suspicion,
while love and compassion, a sense of universal responsibility
are the sources of peace and happiness.
If the situation was such that there was only one learned lama or genuine practitioner alive, a person whose death would cause the whole of Tibet to lose all hope of keeping its Buddhist way of life, then it is conceivable that in order to protect that one person it might be justified for one or 10 enemies to be eliminated if there was no other way.