What exactly makes killing another human being morally unacceptable on at least certain situations (as it clearly is)?

I've phrased the question this way to suggest that I'm not contemplating the viewpoint that killing another human being is morally unacceptable in all circumstances.

I assume that there are some morally justifiable killings (for example, in self-defense when there are no other options for disarming one's attacker).

When it's evident what's wrong with killing in certain situations, it should be easier to explain why it isn't in others.

Applying my answer to cases of voluntary and non-voluntary euthanasia, but light will be shed on whether and, if so, why it is wrong to kill oneself, unborn human beings, and non-human animals, particularly those whose lives are most similar to human lives.