They aren't "normal, everyday" people though are they.
I think we would have to have some definitions, before we could decide that... but, I suspect that, tomorrow, I (or any of us) could be sitting on the Metro, next to such a person, and, not be aware of their potential.
Literally by definition they aren't not 'normal'.
If they were, stabbing children would be normal.
I eat meat. My neighbour is a butcher.
If we lived in an society where everyone else was vegetarian I think my neighbour and I would elicit the kind of horror and cries of "I don't understand" expressed above.
My point is that it is really not too difficult to "other" sentient creatures to the point where we barbecue them.
I was not specifically referring to the stabbing of children. I mentioned Southport only in the context that “evil” was being discussed in that thread. But, I think a clever person like you knew that.
You were talking about "normal" people committing "some terrible acts, against large numbers of their fellow human beings".
Again, these people are not "normal". They are exceptions. If it was "normal" it would not be worthy of comment.
I have never understood why we expect evil people to appear to be a different sort of person to the rest of us. We all have the capacity for good and bad acts to different extents, and luckily evil is not the norm, but one end of the normal distribution of human behavior.
The concept is perhaps made more complicated by the arguments over nature/nurture but that does not mean that evil is an unnatural phenomenon in humans.
Ra, Ra, Rasputin...