There is no single thing that makes murder wrong. And murder may not be wrong in the case of people who cause vast evil but I set such cases aside since they are not the ones you are mainly thinking of. At least that's my impression.
Walter Sinnott-Armstrong and Franklin G Miller explain two grounds on which murder is, or might plausibly be considered to be, wrong.
Murder, disrespect and autonomy
Imagine that Abe robs Betty and shoots her in the
head so that she will not testify against him if he is
caught. As a result, Betty dies. It is clearly immoral
for Abe to shoot Betty. Why?
The most general explanation is that Abe harmed
Betty - his act resulted in bad effects for Betty.
Other explanations are possible, of course. Some
theorists might claim that what makes Abe's act
wrong is Abe's intention, but the reason why Abe's
intention makes his act wrong is that it was an
intention to cause harm to Betty, so the wrongness
of the intention is still grounded in the badness of
the effect that was intended. Other theorists might
instead say that Abe violates Betty's rights, but her
violated right in this case is a right not to be
harmed, so again the bottom line is about harm.
Still others might propose that Abe shows disrespect
for Betty's autonomy or personhood, but what makes his act disrespectful is that it inflicts a loss of
autonomy, and a loss of autonomy is a kind of
harm, broadly construed, so what makes killing
wrong is still that Abe's act had some harmful or
bad effect on Betty.
Nonetheless, it is not enough to say that
Abe harmed Betty We still need to know which
kinds of effects count as harms. That question is
not simple .... Another reason is that,
even if the fact that Abe harmed Betty explains why
his act was wrong, it does not explain how wrong
it was - its degree of wrongness. After all, some
harms are minor. To fully explain what was wrong
with Abe's act, we need an explanation that
captures the full extent of what was wrong with
*Which effect explains that? Abe's act causes at
least two effects on Betty One is death - the loss of
Murder and total disability
The other effect, which is less often noticed, is
total disability. Shooting Betty makes her unable to
do anything, including walking, talking, and even
thinking and feeling. Since Betty then lacks all
abilities to act or do anything, and we are
concerned here only with abilities to act or do
things, Betty's disability is universal. Of course,
anaesthesia can also cause universal disability for
a short time. In contrast, the universal disability
that Abe's shooting causes is also irreversible. Universal and irreversible disability will be called
Which of these consequences - death or total
disability - makes Abe's act of shooting immoral?
Two answers are possible. In one view, Abe's act is
immoral because this shooting causes death, so it is
an act of killing, and killing is immoral unless it is
justified, which it is not in this case. In another
view, Abe's act is immoral because it causes total
disability, so it is an act of total disabling, and total
disabling is immoral unless it is justified, which it is
not in this case.1 These two views are rarely sepa-
rated, because to kill normal people like Betty is to
disable them totally Conversely, there was no way
to totally disable Betty without killing her prior to
the advent of the intensive care units in which the
lives of totally disabled people can be sustained by
mechanical ventilation and artificial hydration
and nutrition along with other techniques. Nonetheless, these views remain distinct, because today
Abe can totally disable Betty without killing her.
He can shoot her in the head so as to cause irreversible brain damage that makes her unable to
walk, talk and even think and feel without also
causing her death, because her life can be sustained
Walter Sinnott-Armstrong and Franklin G Miller, 'What makes killing wrong?', Journal of Medical Ethics, Vol. 39, No. 1 (January 2013), pp. 3-7: 3.
R. E. Ewin, 'What Is Wrong with Killing People?', The Philosophical Quarterly (1950-), Vol. 22, No. 87 (Apr., 1972), pp. 126-139. This article considers a range of other approaches, including some based on utilitarianism.