Those who say that men are led by interest are knaves.
A knavish character will often say - 'Of what interest is it to me to do so-and-so?'
I answer - 'Of none at all, but on the contrary, as you well know. It is of malice and envy that you have done this: hence I am aware of you, because I know that you act, not from interest, but from malice - even to your destruction'.
William Blake - from Descriptive Catalogue (re-punctuated and emphasis added)
So many people, so often, regard self-interest as the prime evil, and try to explain the evil in this world as due to persons and institutions motivated by self-interest...
Everything gets explained that way; so people will seek-out why such and such an action benefits the person that did it. And when this isn't obvious, then remote and indirect self-interest will be wheeled-out.
Contrariwise; if self-interest is not obvious - or if the chain of explanation is disbelieved - then it is assumed that there was no evil but merely some inexplicable coincidence, or incompetence - bad things are confidently ascribed to the sheer 'randomness' and uncontrollability of things.
Those who do this are what Blake termed knaves - cunning, dishonest, deceitful, cowardly, traitorous. In other words knaves are themselves those who are motivated by malice, by spite, by their taking of pleasure in the misery of others.
Northrop Frye (in Fearful Symmetry p56-7) amplifies Blake's passage thus:
By turning away from the world to be perceived we develop an imaginative idleness which spreads a sickness and lassitude over the whole soul, and all vices spring from this... Murder is obviously an expression of the same death-impulse that suicide is, and all evil acts are more or less murderous...
This death-impulse, this perverted wish to cut down and restrict the scope of life, is the touchstone not only of all the obvious vices, but of many acts often not classified as such; like teasing, instilling fear or discouragement, or exacting unthinking obedience.
It is quite inadequate to call self-interest a motive of evil conduct, though the death-impulse may be disguised in that form. Self-interest implies a good deal of control: in all extreme vices there is a mania in which one is hagridden by a 'ruling passion'.
As so often, children understand perfectly that evil is spite and malice; it is only in a culture so sophomoric, so adolescent, so knavish as ours - that we claim to see-through the 'obvious (and true) explanation such as to regard evil as an expression of mere self-interest (most often specifically economic self-interest).
Yet self-interest is universal - and so is no explanation at all; especially in a world where 'goodness' is defined in 'utilitarian' terms of publicly-observable and quantifiable 'altruism' (e.g. raising money for 'charity') - such that altruism is itself the grossest form of self-interest.
Furthermore, the focus on self-interest serves to disguise real evil; because the 'mania' that drives a 'hagridden' doer of malicious evil will often bring about his downfall - and in modern culture that makes him a 'victim', worthy of sympathy. Instead of seeing self-destruction as a hallmark of the murderous nature of true evil, it engages our sympathy - and thus we are corrupted.
This is vital to bear in mind when so much of modern evil is bureaucratic, such that responsibility for evil is eluded, and we seldom know even the identities of those whose malice drives the evil. But, whether we know them or not, we can be sure that they are there.