Be it some insignificant “little Stalin” buried deep in the bureaucracy or one of the famous evil leaders of history, how is one to explain the corruption of a person who has the power and thus the choice to improve the world but instead not just abuses power but does so eagerly? In Notes from Underground, Dostoevsky appears to be offering the ravings of a lunatic but is in fact exposing the disease in the souls of all who have been corrupted by power.
кто же может своими же болезнями тщеславиться, да еще ими форсить?
Впрочем, что ж я? — все это делают; болезнями-то и тщеславятся [ru.wikisource.org.]
People do pride themselves on their diseases. [Notes from Underground (Russian Classics in Russian and English), 13]
Человеку надо — одного только самостоятельного хотенья, чего бы эта самостоятельность ни стоила и к чему бы ни привела. [ru.wikisource.org.]
What man wants is independent volition, whatever that independence may cost and wherever it may lead. 
People are not “rational” because what they value is not that which is good nor even that which benefits themselves; what they value above all is the freedom to make their own decisions. It follows that they will then take pride in those decisions–regardless of the justice, utility, or decency of those decisions–simply because they made the decisions themselves. Therefore, no matter how nasty they may be, no matter how morally diseased they are, they take pride in their own chosen paths – good or bad, choosing your own path defines you as an individual, and that individualism is, to humans, the highest good.
Who has not known low-level managers promoted beyond their level of competence who spent their time “setting fires in order to play fireman,” with that concept typically taking the form of finding a scapegoat to blame, destroying the scapegoat’s career, and taking the credit for “saving the institution.” You may find that repulsive, but to the corrupt manager, such behavior constitutes the very proof of his individuality. And that is of course all the more the case for the warmongering politician, terrorist leader, or endlessly acquisitive billionaire. Dostoevsky had it all figured out long before Stalin became the classic example, before anyone tried to exterminate the Palestinians, before the Empire destroyed Fallujah.