Reason

A little Kant - Introduction and part of chapter one from The Critique of Practical Reason. This is a good starting point for an intelligent consideration of the reason of murder, morality, and by extension, law. Read more, including some neat discussion on the nature of good and evil, here.
Interestingly, I didn't see much about causing pain to another as a motivation for action or an object of desire. We could extend the notion of "pleasure" to the notion of satisfaction (realized or believed) gained at harming another. Of course, that is just seriously fucked up, but I suggest, purely and only for the sake of argument, entertaining the notion that any form of reason or morality would allow for murderous impulses as anything other than blatantly wrong. Without that allowance, there is either no argument at all or an entirely different kind of argument.

1788
THE CRITIQUE OF PRACTICAL REASON
by Immanuel Kant
translated by Thomas Kingsmill Abbott
FIRST PART.

INTRODUCTION.
Of the Idea of a Critique of Practical Reason.

The theoretical use of reason was concerned with objects of the
cognitive faculty only, and a critical examination of it with
reference to this use applied properly only to the pure faculty of
cognition; because this raised the suspicion, which was afterwards
confirmed, that it might easily pass beyond its limits, and be lost
among unattainable objects, or even contradictory notions. It is quite
different with the practical use of reason. In this, reason is
concerned with the grounds of determination of the will, which is a
faculty either to produce objects corresponding to ideas, or to
determine ourselves to the effecting of such objects (whether the
physical power is sufficient or not); that is, to determine our
causality. For here, reason can at least attain so far as to determine
the will, and has always objective reality in so far as it is the
volition only that is in question. The first question here then is
whether pure reason of itself alone suffices to determine the will, or
whether it can be a ground of determination only as dependent on
empirical conditions. Now, here there comes in a notion of causality
justified by the critique of the pure reason, although not capable
of being presented empirically, viz., that of freedom; and if we can
now discover means of proving that this property does in fact belong
to the human will (and so to the will of all rational beings), then it
will not only be shown that pure reason can be practical, but that
it alone, and not reason empirically limited, is indubitably
practical; consequently, we shall have to make a critical examination,
not of pure practical reason, but only of practical reason
generally. For when once pure reason is shown to exist, it needs no
critical examination. For reason itself contains the standard for
the critical examination of every use of it. The critique, then, of
practical reason generally is bound to prevent the empirically
conditioned reason from claiming exclusively to furnish the ground
of determination of the will. If it is proved that there is a
[practical] reason, its employment is alone immanent; the
empirically conditioned use, which claims supremacy, is on the
contrary transcendent and expresses itself in demands and precepts
which go quite beyond its sphere. This is just the opposite of what
might be said of pure reason in its speculative employment.
However, as it is still pure reason, the knowledge of which is
here the foundation of its practical employment, the general outline
of the classification of a critique of practical reason must be
arranged in accordance with that of the speculative. We must, then,
have the Elements and the Methodology of it; and in the former an
Analytic as the rule of truth, and a Dialectic as the exposition and
dissolution of the illusion in the judgements of practical reason. But
the order in the subdivision of the Analytic will be the reverse of
that in the critique of the pure speculative reason. For, in the
present case, we shall commence with the principles and proceed to the
concepts, and only then, if possible, to the senses; whereas in the
case of the speculative reason we began with the senses and had to end
with the principles. The reason of this lies again in this: that now
we have to do with a will, and have to consider reason, not in its
relation to objects, but to this will and its causality. We must,
then, begin with the principles of a causality not empirically
conditioned, after which the attempt can be made to establish our
notions of the determining grounds of such a will, of their
application to objects, and finally to the subject and its sense
faculty. We necessarily begin with the law of causality from
freedom, that is, with a pure practical principle, and this determines
the objects to which alone it can be applied.


ELEMENTS OF PURE PRACTICAL REASON.
BOOK I. The Analytic of Pure Practical Reason.
CHAPTER I. Of the Principles of Pure Practical Reason.

I. DEFINITION.
Practical principles are propositions which contain a general
determination of the will, having under it several practical rules.
They are subjective, or maxims, when the condition is regarded by
the subject as valid only for his own will, but are objective, or
practical laws, when the condition is recognized as objective, that
is, valid for the will of every rational being.

REMARK.
Supposing that pure reason contains in itself a practical motive,
that is, one adequate to determine the will, then there are
practical laws; otherwise all practical principles will be mere
maxims. In case the will of a rational being is pathologically
affected, there may occur a conflict of the maxims with the
practical laws recognized by itself. For example, one may make it
his maxim to let no injury pass unrevenged, and yet he may see that
this is not a practical law, but only his own maxim; that, on the
contrary, regarded as being in one and the same maxim a rule for the
will of every rational being, it must contradict itself. In natural
philosophy the principles of what happens, e.g., the principle of
equality of action and reaction in the communication of motion) are at
the same time laws of nature; for the use of reason there is
theoretical and determined by the nature of the object. In practical
philosophy, i.e., that which has to do only with the grounds of
determination of the will, the principles which a man makes for
himself are not laws by which one is inevitably bound; because
reason in practical matters has to do with the subject, namely, with
the faculty of desire, the special character of which may occasion
variety in the rule. The practical rule is always a product of reason,
because it prescribes action as a means to the effect. But in the case
of a being with whom reason does not of itself determine the will,
this rule is an imperative, i.e., a rule characterized by "shall,"
which expresses the objective necessitation of the action and
signifies that, if reason completely determined the will, the action
would inevitably take place according to this rule. Imperatives,
therefore, are objectively valid, and are quite distinct from
maxims, which are subjective principles. The former either determine
the conditions of the causality of the rational being as an
efficient cause, i.e., merely in reference to the effect and the means
of attaining it; or they determine the will only, whether it is
adequate to the effect or not. The former would be hypothetical
imperatives, and contain mere precepts of skill; the latter, on the
contrary, would be categorical, and would alone be practical laws.
Thus maxims are principles, but not imperatives. Imperatives
themselves, however, when they are conditional (i.e., do not determine
the will simply as will, but only in respect to a desired effect, that
is, when they are hypothetical imperatives), are practical precepts
but not laws. Laws must be sufficient to determine the will as will,
even before I ask whether I have power sufficient for a desired
effect, or the means necessary to produce it; hence they are
categorical: otherwise they are not laws at all, because the necessity
is wanting, which, if it is to be practical, must be independent of
conditions which are pathological and are therefore only
contingently connected with the will. Tell a man, for example, that he
must be industrious and thrifty in youth, in order that he may not
want in old age; this is a correct and important practical precept
of the will. But it is easy to see that in this case the will is
directed to something else which it is presupposed that it desires;
and as to this desire, we must leave it to the actor himself whether
he looks forward to other resources than those of his own acquisition,
or does not expect to be old, or thinks that in case of future
necessity he will be able to make shift with little. Reason, from
which alone can spring a rule involving necessity, does, indeed,
give necessity to this precept (else it would not be an imperative),
but this is a necessity dependent on subjective conditions, and cannot
be supposed in the same degree in all subjects. But that reason may
give laws it is necessary that it should only need to presuppose
itself, because rules are objectively and universally valid only
when they hold without any contingent subjective conditions, which
distinguish one rational being from another. Now tell a man that he
should never make a deceitful promise, this is a rule which only
concerns his will, whether the purposes he may have can be attained
thereby or not; it is the volition only which is to be determined a
priori by that rule. If now it is found that this rule is
practically right, then it is a law, because it is a categorical
imperative. Thus, practical laws refer to the will only, without
considering what is attained by its causality, and we may disregard
this latter (as belonging to the world of sense) in order to have them
quite pure.

II. THEOREM I.
All practical principles which presuppose an object (matter) of
the faculty of desire as the ground of determination of the will are
empirical and can furnish no practical laws.
By the matter of the faculty of desire I mean an object the
realization of which is desired. Now, if the desire for this object
precedes the practical rule and is the condition of our making it a
principle, then I say (in the first place) this principle is in that
case wholly empirical, for then what determines the choice is the idea
of an object and that relation of this idea to the subject by which
its faculty of desire is determined to its realization. Such a
relation to the subject is called the pleasure in the realization of
an object. This, then, must be presupposed as a condition of the
possibility of determination of the will. But it is impossible to know
a priori of any idea of an object whether it will be connected with
pleasure or pain, or be indifferent. In such cases, therefore, the
determining principle of the choice must be empirical and,
therefore, also the practical material principle which presupposes
it as a condition.
In the second place, since susceptibility to a pleasure or pain
can be known only empirically and cannot hold in the same degree for
all rational beings, a principle which is based on this subjective
condition may serve indeed as a maxim for the subject which
possesses this susceptibility, but not as a law even to him (because
it is wanting in objective necessity, which must be recognized a
priori); it follows, therefore, that such a principle can never
furnish a practical law.