Umut asked:

When does a human being has right to kill another human being? To make the question a bit
interesting I set up the following case study. There are 3 people in the scene: an observer, an adult
and a child. The observer sees that the adult person is about to kill the child. The observer has to
make a choice. Either the observer watches the kid to get murdered or he/she kills the adult person. If
the observer thinks that the adult does not have the right to kill the child, he should kill the adult but in
this case this becomes a paradox since the observer does not have the right to kill the adult either.

What does a professional philosopher say about this?

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I do not see a paradox in the situation you have described. Your question, 'When does a human
being have the right to kill another human being?' implies that there are situations in which killing is
justified, and other situations in which killing is not justified.

An adult is about to kill a child. For the sake of the example, we may assume that this killing is a
wrongful killing. It is not a case, for example, of turning off a life support machine, after a court order
has given permission to do so.

The second adult sees that this wrongful killing is about to take place. The second adult is therefore
morally justified in taking the minimalsteps necessary to prevent the killing. If there is no reasonable
alternative but to take the first adult's life (for example, if the second adult is a long way off and armed
with a pistol, and the first adult has ignored his warning shots) then the second adult is justified in
shooting, and if necessary killing the first adult, because it prevented a wrongful killing from taking
place.

There is an apparent paradox, however, for any person who believes that killing is not justified under
any conceivable circumstances. Someone who holds this extreme pacifist view can still defend their
decision not to shoot the first adult along the following lines: There is a morally significant difference
between the actions I intentionally do, and the actions that other persons do as the unintended
consequence of my actions. Though I am to some extent responsible for all the foreseeable
consequences of my actions, whether I intend them or not, my responsibility is less - other things
being equal - if these consequences result from the wrong doing of others that could only be
prevented by my own wrong doing.

Geoffrey Klempner