I don’t think the question here was about the guy being about to kill you. It was specifically about him being harmless at the moment. When someone is about to kill you there is adrenalin, and the will to survive overpowers your natural inhibitions. The topic here was about minus this element would you bring yourself to kill based on knowledge. It is an interesting topic, but the Halacha might change accordingly.
That’s interesting about the Mosar, although there is still a difference. The Mosar is constantly conniving and collecting information, so he is active. The serial killer, on the other hand, is someone who will difinitely kill but not everyone every day. He can wake up one day and decide that it is a good day for killing or he might do it on the third Monday of every even month. Either way, he is not, in any way, active right now. I could hear, though, that you can look at him like a wild tractor. Now that he is a Muad or Muchzak he is always heading towards his next act.