When speaking of moral evil we must distinguish the subject from several closely related terms. Moral evil is distinct from natural evil (things such as famines or plagues or tidal waves), in that the latter is an objective state of affairs that depends, not on a person, but on the course of nature. No aspect of the course of nature can be said to be, in itself, morally evil (unless, perhaps, the pantheists are right in saying that all things are God; but then, so much the worse for their explanations). One might call this natural evil physical evil. Moral evil is set apart from physical evil, or natural evil, in that there must be a will involved. All matters that fall within the moral scope do so by virtue of being related to will, that chief faculty of man, which works in concert with the faculty of reason to direct and bring about ordered (or disordered) activity. Hence, where there is a will there is a way (and that way will always, of necessity, have a moral character).
But in what consists the evil of a moral act? (If one does not like the word “evil” then “badness will suffice”, given that it is understood as being opposed to rightness, which is correct ordering to the good, which brings us to the same concept in a round-about manner). It turns out that we cannot say that an act is evil objectively, that is, CONSIDERED IN ITSELF the act AS SUCH cannot be what is evil. Take for example my murdering someone by shooting them. The actual activity, the physical happening, of the shooting cannot be what is evil in this morally evil act. This might sound strange but it can be demonstrated. Contrast my action in the murder with my action as a soldier on the battlefield. In war I can commit a token of the same type of action, yet without the moral evil. I can, presuming the war is just, kill an enemy by firing upon him with my rifle (or what have you). Hence we can see that it cannot be the positive, physical, aspect of the moral action which is good or evil, TAKEN ABSTRACTLY. That is, in itself, qua physical action, as such, in the abstract, considered apart from the agent, there is no evil involved. So where is the evil in morally evil acts? (And, we must remember, that it is activity that morality is concerned with). Moral evil consists in, primarily, a privation of right order in the human will towards a harmony with right reason. When there is such a privation there is evil action. Why? Because of the natural law of course. The natural law is an objective law and hence we are able to judge our activities against this ordering of right reason, given for the direction of the common good, whereby our actions relate by degrees of intelligibility and reasonableness. (Don’t stress the word “degrees” too much).
We can see, then, that an agent’s intention cannot alter the objective characteristics of an action in relation to the natural law, although it can, in some circumstances, obviate moral culpability.