I've always marveled at people's insistence on not believing that others can cultivate and act on bad/evil impulses without necessarily being insane. That insistence also pushes a simplistic view of mental illness: suffering from any of the numerous conditions that fall under it doesn't automatically equate to loss of awareness of self/propriety, societal mores, etc. And even in acute cases involving reduced consciousness/self-awareness, it doesn't mean that it occurs 24-7 or crucially, during the planning and execution of a crime.

I was also struck by the noted increase in certain types of “single-issue” terrorism:

"Last year the director of the FBI, Louis Freeh, told Congress, “The most recognizable single issue terrorists at the present time are those involved in the violent animal rights, anti-abortion, and environmental protection movements. … the potential for destruction has increased as terrorists have turned toward large improvised explosive devices to inflict maximum damage.”