It is not just fighting monsters.
When we concentrate on a value system, or a particular set of human behaviors, we tend to make our focus more and more narrow, in order to understand the specific thing we seek to understand. But human behavior is broad, varied, and complex. So, if I seek to understand the minds of murderers, I must look at the lives of murderers, and their opinions.
They are humans, and I too am human. What motivates them is also a mutilator to me, on at least some level. The more closely I study murder, as a behavior of humans, the more I can see the same human characteristics in myself, and in you, and in all other people. Eventually, I empathize with murderers.
So, I become more able to feel, and identify with the aberration of murder than with the more common, and more adaptive behaviors of "normal" people, because I have engaged in the aberrant behavior of murder vicariously to such an extent that it has become more normal to me than the virtue of the more common man. I distrust his virtue, because I cannot understand it.
I have studied evil, but I have not studied virtue.
I have looked into the abyss, and in the end, I peer out of it myself.