How do you know there were signs? How do YOU know that steps were not taken to insure this man was who he said he was. For all we know, even his fiance was unaware of what he was capable of. You don't know the specifics so you shouldn't run your mouth.
You think that this is a perfectly white world, where everyone will just show their true colors? Do you think that every background check will show every detail of your life? You think life is just handed to you, and that as long as you do your best, everything will come out right in the end? The killer will be caught before he does the crime?
Well,"In my world, everyone's a pony and they all eat rainbows and shit butterflies!"
You really need to get out more, and learn the truth about the world and the human species, before you try and run a thread into the ground, with pointless nonsense.