We can 'know about' something in a detached and observational way; but full knowledge of a thing entails experience of that thing, and by experience I mean empathic identification with it - which entails the inner experience of thinking and of feeling emotions at the most primary level.
Therefore, we should not try to understand evil - because to understand it we must become it; and that might have a lasting, even permanent, effect.
We need to know about evil sufficiently to resist it - but that is all. And that is not much, because the Good man can recognise evil without requiring to know much about it - so we ought to strive to be Good much more assiduously than we try to understand evil.
Evil, of course, is always telling us we don't understand it, and therefore really ought to go into it more fully and deeply - ninety-something-percent of the modern mass media - including many of the more prestigious novels movies and TV shows - are indeed designed to lead us into a deep identification with evil... ask yourself, why would that be, if knowing evil really was helpful?
Are the people who make and promote and admire these media the kind of people who would really want to aid the battle for Good? - or are they instead the kind of people who would want to subvert the Good? To ask is to answer.
The Good man is always, therefore, accused of ignorance, of naiveté, of not really understanding evil as evidence by his failure to describe evil such that people are drawn into empathic identification - how often have you heard Tolkien accused of this, or Chesterton, or anyone whose character or work is morally exemplary?
And indeed there is not much to understand about evil because it has no positive qualities - deep down, at root, evil is merely against-Good; and the varieties of evil are merely due to opposing the various subtypes of Good - unity, virtue, beauty, truth and their subdivisions.
That is all we need to know, and all that we should strive to know,
Evil is something to be resisted, fought or shunned - but not to be understood.