I do not subscribe to the view that people who do terrible things believe their actions are necessary for a greater good. I think many such people go out of their way to inflict misery and take a perverse pleasure in their power to do so. We often have to endure a high concentration of these people in high places because they have had a lifelong obsession with power and the unyielding desire to wield it over others, with no moral qualms about destroying anyone deemed a threat to their manipulations.
It is fashionable to try to explain wickedness as the result of a series of rational decisions based on a certain set of beliefs, such as by those who perpetrate atrocities in the name of religion, or rulers who commit genocide and other horrors – and in a way it is heartwarming that proponents do not understand the nature of evil or what it does; but they should realise that the subjects of their empathy, underneath often charming facades, think of them as weak dupes to be taken advantage of and abused.
To describe evil-inclined people as ill, and that some were tragically born with a high susceptibility to the disease, is nearer to the truth.