It might seem like you would have absolutely no interest whatsoever in taking the necessary steps that would be required of you to sympathize with mass murderers. After all, the crimes they commit are so atrocious, that no one would blame you if you were completely uninterested in trying to understand them and figure out why they did what they did.
But, if you are someone who is interested in the communities of mental health, scientific discussion, and psychological research, you might be interested in sympathizing with mass murderers in the sense that you would only ever be doing so out of an interest in finding out the ways in which they think so you can help make strides towards treatment for people who might show warning signs about thinking the way mass murderers do in the future.
What a great deal of scientific research seems to indicate is that mass murderers have a deficiency within their amygdala, which is the emotional part of the brain.
If this proves to be true, it would certainly explain quite a bit about why mass murderers act they way they do, which is to say impulsively. These are, of course, not in the same category as sociopaths who are more cold and calculating.