enjoyable but far fetched

I have finished watching the first three installments of the 'Messiah' series on DVD. I'm enjoying it, but it can also be placed in that rather long and irritating tradition of police movies in which the villain is somewhat superhuman. The second episode ('Vengeance is Mine') is the most far fetched for me. How on earth could the villain of the story, a rather vulnerable looking female clerk who apparently has not trained as a ninja or anything, manage to attack and incapacitate so many young and healthy people in public places and then leave them to die a horrible death without anyone noticing until it's too late? The movie does not explain in any way how she pulls off this amazing feat. We're supposed to just believe she was, somehow, able to. At least in the first in the series, a kind of a credible explanation is provided as to how the villain managed to get access to his victims.