Rarity of the Human World I've read in Buddhist texts that being born into the human world is a rare and privileged event.

If that's the case, then why are killers and "bad" people allowed to be born into the human world?  

I'm not sure if Buddhists teaching elaborate on the ability to predict a person's future actions but it is my belief and understanding that a person's past karma can foreshadow such a thing.
The word translated as 'human' namely, 'manussa' means 'high minded'. It does not mean a 'homo sapien'. SN 56.47 describes the meaning of what is not the human state, as follows:  


  Sooner, I say, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole
  than the fool who has gone once to the nether world would regain the
  human state. For what reason? Because here, bhikkhus...
  
  ...there is no conduct guided by the Dhamma, no righteous conduct, no wholesome activity, no meritorious activity. Here there prevails
  mutual devouring, the devouring of the weak. For what reason? Because,
  bhikkhus, they have not seen the Four Noble Truths. What four? The
  noble truth of suffering … the noble truth of the way leading to the
  cessation of suffering.


Therefore, it can be discerned quite plainly, without any dispute or contraversy, the human state refers to living a harmless righteous enlightened life, which is indeed extremely rare in this world of people devouring, exploiting, abusing, raping &amp; harming themselves &amp; each other.