Not sure as to the train of thought of the question, or the logic within...
So, because wife of serial killers do not know their husbands, that means, nobody else knows their respective partner? That, for one...
This kind of thinking assumes that the individual is the point of reference of the whole. This kind of thinking is the main reason great empires (aka the United States) have such an ordeal within the United Nations, or in their international relations -they assume everybody else thinks/knows what they know, and that there is no other knowing beyond theirs.
My answer/opinion here (and this question is in essence very related to the matter of knowing oneself) is that, well, if one barely gets to know oneself during one's lifetime, how do we expect knowing the other fully?
Is it a matter of knowing really?
There's a great book on the matter: The Little Prince. It relates with precious stories how does one dwell a lifetime in trying to know in order to acquire some sense of security and control.
But misses out on living Life.
I say do not buy fear. Why dwell on terror? Not to be oblivious on the fact that bad things might and do happen, but we have real problems in our daily to spend time trying to solve imaginary ones.
Live Love Laugh... and down with the serial killers, all dementors and toxic persons for any matter!
Have a blessed day!