Who does more damage, A) mean-spirited people or B) "normal" people acting thoughtlessly? According to Hannah Arendt, the answer is clearly B. I would agree. Why? Because we serve as our own gate-keeper as to what what aspects of the world are relevant, usually oblivious to the fact that the "gate-keeper" of the flow of "relevant" facts is our sycophantic enabler, and that the gatekeeper is often willing to help us express our deepest darkest instincts.
How is it that “normal” people so often behave (and vote) as moral monsters? In Eichmann in Jerusalem (discussed below), Arendt has written that the "banality of evil," the failure to think, leads to monstrous deeds--the road to hell is mostly paved with a lack of intentions. I largely concur with Arendt, but I would explain the source of most evil in terms of the psychological concept of attention: human animals have limited attentional capacities, and ghastly things can happen when this scarce human resource (the ability to attend) is diverted (often self-diverted). Moral monsters self-train themselves to pre-filter their sensory perceptions so that they don't need to attend to anything in the world that challenges their preferred viewpoints.
The trick to becoming a banally evil person is to allow yourself to dwell on limited viewpoints and experience. To grow your evilness, stop being self-critical, stop being skeptical and stop exposing yourself to viewpoints that challenge the way you currently live your life. When you become a professional at selectively attending to the "things" of the world, you can feel the rush of becoming a self-certain--you'll become so certain of your beliefs that you won't hesitate to impose your narrow intellect onto everything and everyone you encounter. And even when you are incredibly wrong-headed, you won't realize it, thanks to the Dunning-Kruger effect. That is the great power of the ability to selectively attend to one's favorite parts of the world.
It takes courage to expose one’s self to information that challenges one’s pre-existing beliefs. Humans are intrinsically able to be self-manipulative--being skeptical requires much more work than running with the types of believes and conclusions that have pleased us in the past. That is also the nature of the confirmation bias. Most of us, most of the time, sub-consciously (or semi-consciously) selectively expose ourselves mainly to the types of information that will substantiate our preconceived notions and motives. We’ve all seen this with the many dysfunctional people who use the Internet selectively. They seek out only web sites that are compatible with their pre-existing bigoted, consumerist or shallow life-styles.
If you put on blinders that allow you to see only a limited slice of the world around you, you can spare yourself the need of emotionally reacting to desperate needs of humans around you. Most of us constantly blind ourselves to the plight of starving children in Africa. Out of sight, out of mind. It’s merely a matter of diverting our attention to something else, something not so disturbing.