You are correct, methinks. People who cannot endure the atrocities of others, in their hearts of goodness, always succumb to the idea that those who are so cruel are mad or insane.
May we always remember that perfectly rational persons can be cruel and crush your child like an ant under their heel, not because they are insane, but because they are cruel.
There is another factor, too: In the past two generations, Western culture has tended to discount and discredit the existence of Evil and evil people. This plays into the tendency to succumb to the "mad/insane" characterization all too easily.