The world being what it is, we sometimes have to choose between two less than desirable options. The problem with choosing the lesser of two evils that are distasteful enough is that one has to numb their conscience to do it. This is what is known as a ‘Faustian bargain.’ We may lie to others and especially ourselves about it, but there it is.
But the individual will find that if they choose to make this kind of rotten deal with their own conscience enough times, and put their inner voice aside to make an expedient, practical choice rather than a moral one, and pile lie upon lie to ease the pain of their betrayal, eventually their conscience will fail them when it matters the most. Or they may simply find themselves too compromised to do anything but overtly choose what is objectively wrong or suffer serious personal consequences. They have intertwined their fingers with the power of darkness so deeply that now they are his.
And this is how the banal, time serving functionaries familiarize themselves into eventually doing otherwise unthinkable, and sometimes monstrous things.