The only good is knowledge and the only evil is ignorance. - Socrates as quoted in Diogenes Laertius' Lives of Eminent PhilosophersIs Evil-Doing Madness? Underlying much comment on the horrendous genocides that continue into the present day appears to be the assumption that rationally acting persons could not deliberately choose to behave "evilly," no matter what we might take to be their values.
What are we to say about madness when it takes hold of an entire society? How are we to conceptualize madness that becomes normative within a particular culture?-- R. Koenigsberg ((March 3, 2014)) Social Madness/Collective Delusion 
Thus, if, as many believe, God is the model of goodness and rationality -- The Book of Job notwithstanding -- then when people pursue "evil," they must be "mad," or "irrational," at least. This assumption that "rational" people never -- by definition, as it were -- consciously, deliberately pursue evil is a "fantasy" which derives much earlier than from much faulted Enlightenment philosophers. (See Koenigsberg to the contrary, cited.)
Is Irrationality a Mental Condition? If you take something like "all carefully considered action is rational" to be something like Newton's First Law of Motion, then it is not so much descriptive of actual but of ideal behavior. Newton's laws were applied to objects which are idealized as single point masses. They were meant to provide possibilities for investigating deviations from the (axiomatic) ideals postulated. The identification of Good with Rationality is part of a long tradition and is reflected in the Goya etching reproduced to the right of this text.
For example, we may say upon finding out a friend has started using heroin, "How is that going to solve anything?" In doing so we have already formulated a hypothesis "explaining" his behavior: he is trying to stand up to stress, perhaps; or, to forget what bothers him, etc. We try to understand the drug-user's behavior at somehow rational, even if ineffective in the long run. But if we find he lives a generally unproblematic life, full of happy occasions, we might wonder at the "irrationality" of his action. Perhaps he "needs help," whether he thinks he does or not!
There's a real issue. People thought to be "really irrational" risk losing their freedom and even, their possessions. For example, many people advocate compulsory supervision of the homeless, particularly when the weather is cold. To these would be controllers -- generally described in the media as "saviors" or "helpers" -- it is irrational to sleep out under a bridge in a cardboard box, when offered the hospitality of a "shelter." (See also Are Humans Rational? What’s At Stake?)
Can We Agree on What is Good? In order to bypass a long controversial discussion about what "really" is good, we adapt a framework such as that suggested by Paul Ziff , i.e. "good" means "answering certain interests." Thus, we can understand how a person who advocates strong control of firearms can still discriminate between, say, a "good gun" and an "inferior gun." More important, we can understand how a rational person can pursue evil. This is nothing new or startling. We all understand someone making a choice for the least evil of offered options. And we don't normally regard such as "irrational." (See Choosing The Lesser Evil: a Moral Failure?)
(Idealists will likely find Ziff's analysis bothersome. It gives up on trying to discover what is "really" good, focussing on how people actually use the word in practical situations. It does not try to disregard the existence of conflicting "goods" since it concedes the existence of possibly conflicting interests.)
Our modern hope, (fantasy?) is that the spread of knowledge, i.e. the near universal acceptance of certain privileged sources of belief -- e.g. Science, less likely, Religion -- will forestall conflicts caused by the rational pursuit of dominance which can favor a minority of individuals, families, classes, tribes, without necessitating the extirpation of those less powerful.
Evil-Doing is not, per se, Irrational. The Nazis were generally not insane; nor were their enthusiasts or followers. Nor are the many people we deign to call criminals, perverts, and the like. And it does not follow purely logically that "evil-doers" ought to be punished. This takes additional commitments to values -- some of which great many adhere to -- to make such infliction rational. (See Nassir Ghaemi (2011) A First Rate Madness p.245.)
The attempt by Koenigsberg and others debating this issue is their trying to address -- and even "solve" -- moral problems while disavowing commitments to anything more substantial than a "scientific" pursuit of practical knowledge. But moral issues may not be sufficiently "objective" to permit of such solution.
The Really Basic Problem. Koenigsberg and others professionally concerned with mental health suggest that "irrationality" be redefined from a characteristic of individuals to one of collectivities. But "redefining" irrationality as a group madness is hardly going to make a great dent here. It would most probably only mask the basic problem.
What is this basic problem? Maintaining and recognizing for a selected cadre of professionals the ownership of certain human behavior -- and its treatment rights -- despite their lack of consensus on fundamental ethical warrants, terminology and therapy.
For examples and to examine these issues further, see Can Criminal or Immoral Behavior Be Dealt With Objectively?
See also, Is it Reasonable to be Logical?
 Richard A. Koenigsberg, Ph.D (March 3, 2014) Social Madness/Collective Delusion. LIBRARY OF SOCIAL SCIENCE
 Paul Ziff, "On the Word Good" in Semantic Analysis (1960).