There is a desire to call evil people crazy and/or stupid, though evil people can be both thoroughly sane and rational.
By blaming insanity for evil, you imply it can be eliminated by curing all mental illness and cure the bad person with therapy and drugs.
By blaming stupidity, you imply it is cured by education or easily prevent it through supervision of the retarded.
Evil can be defined as choosing actions that deliberately cause pain and suffering in others, typically for your own enjoyment or benefit. The man who rapes for his sexual pleasure, not caring about her pain; the sadist who tortures others for his pleasure though they are unwilling; the men who join ISIS for permission to kill those they don't like and be rewarded sex slaves as a result; these are evil people.
And while their choices are a decided minority, and we define sanity by the majority, calling them crazy denies the fact that many more people have such impulses they would act out upon if not for vigilance and severe punishment. It is comforting to say it is a small minority capable of such things, but from the Russian gulags to German Holocaust to Cambodian Killing Fields, we have proof every society can produce/release many such people when conditions are right - so it isn't a 1% to be wary of but a potential all people must protect against.
Refusing to admit everyone has that potential is reassuring and denies the individual the need to control themselves.