When we think of great harm, we think of overtly evil people-those who in a truly malevolent fashion derive satisfaction from hurting people. But these people are not responsible for any substantial amount of evil in this world. Left to themselves, their influence does not reach far before good people restrain them. In order to achieve any measurable evil, these people require many henchmen who are willing to tolerate or support them. It is the supporting people—people who are, in their minds, doing far less harm than the overtly malevolent— that cause the greatest harm in the world.
It has been said that the road to hell is paved with good intentions. Whether or not this is true, a similar phrase would be, which is that the road to hell is paved with people who willingly did what they knew was wrong under the first blush of opposition. These are people who cracked under the pressure of losing their job, or disappointing their inlaws, or being ridiculed by their peers. These are the people who are “just doing their job,” or who put up their arms and say “what else could I do? I'm only human.”
If it were possible to sin without incurring lasting effects on your conscience, these people would not be such a threat to the world. But it isn't. Everything conscience-related is cumulative. You cannot deviate in the least from what you know is right without reducing your ability to do or even see what is right in the future.
The road these people willingly descend is a road of enslavement. They accept lightweight cords of bondage, thinking it is easy to break them if the need arises, without realizing that these cords will grow in strength without them noticing (see 2 Nephi 26:22).