Jones believed, or at least said he believed, that because of the shooting of the Congressman (which he claimed he had no control over), they were all doomed. The US government was going to kill them.
I apologize for the source (though I can verify it is accurate, having listened to the recording), but here's the transcript of the tape of Jones telling them they needed to die:
http://creepypasta.wikia.com/wiki/The_Jonestown_Tape
> So my opinion is that you be kind to children and be kind to seniors and take the potion like they used to take in ancient Greece and step over quietly because we are not committing suicide. It's a revolutionary act. We can't go back; they won't leave us alone. They're now going back to tell more lies, which means more congressmen. And there's no way, no way we can survive.
> JONES: Don't be afraid to die. You'll see, there'll be a few people land out here. They'll torture some of our children here. They'll torture our people. They'll torture our seniors. We cannot have this. Are you going to separate yourself from whoever shot the congressman? I don't know who shot him.
Essentially, Jones believed their (in his mind) communist, multiracial utopia was doomed to fiery death the moment his people shot Leo Ryan. The mass suicide (or murder, if you prefer) was in his mind, and in the minds of many of his followers, a protest against the racist capitalists.