In general, "a" is used when the noun is generic or it's not clear which (of many of that noun) 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.
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 pretty 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.
When he then says, "the people" it implies we should know which people he means. We know 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, therefore, 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 (who the rebels are fighting against and who, ironically, we opposed because we thought they were bad people).