In general, "a" is used when the noun is generic or it's not clear which I am referring to.  "The" is used when either referring to something I've already mentioned, or referring to something that the reader should already know.  

For example:

> Mr. Smith drove slowly toward his house.  *The* house was located on top of a small hill.

Here *the* is appropriate because I've already specified which house I mean. 

> Jaundice is a disease of *the* liver.

Here *the* is appropriate because the reader should know that the liver is an organ in the human body.  I don't have to specify which liver, since there's only the one.

> "You know, every time we take rebels, whether it’s in Iraq or anywhere else, we’re arming people. And you know what happens? They end up being worse than the people.”

This sentence is already a vague.  It's unclear what the author means by "take rebels" -- the expected meaning is "regarding rebels" or "looking at rebel groups", but then he implies to "take rebels" means "to **arm** people and make them into rebels".  

It's an unexpected definition, but we can accept it and move on. So when he uses *the* people it implies we should know which people he means.  He's not referring back to "the people who we made into rebels" because he says "they (the rebels) end up being worse than the people".

So we have to read between the lines based on context.  My best guess is "the people" refers to "the people who the rebels are fighting against."  Then the sentence makes more sense:

> They (the rebels who we armed) end up being worse than the people they are fighting against (who, ironically, we opposed because we thought they were bad people).