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 & harming themselves & each other.
"bad" people allowed to be born into the human world
This statement does not make sense. Are you assuming there's some kind of God who is allowing beings to be born as humans?
Just because it is hard to get a human birth, it doesn't mean they are going to act virtuous after being born human. To give a simile, think of it as becoming the president of a country. How hard it is to achieve that? But does it mean all of them are going to act virtuous after coming to power? Becoming a god is even more rare. But look at how the Mara behaves despite being from the highest heaven.
Kamma is not resultant, so while old wholesome kamma making it's resultant arising, the killer can make new unwholesome kamma, together, in the same vithicitta.
Kamma fight kamma to take a occasion to make their resultant arise. In a second we have done millions kamma. And each kamma have their resultant. See 4 kamma in chapter 5th of abhidhammatthasangaha.
Killer maybe born in niraya 500 times, then in peta 500 times, then in tiracchana 500 times, then in cripple 500 times before being as killers, again, in the present life. See dhammapathattakatha.
Kamma is uprightly.
People do good and bad both. People who have done some good kamma is be born as a human. Due to various factors they become killers and bad people.
