From what I've read about the incident, there are some interesting facts regarding this case.
After the incident, investigators found certain drugs(Quaaludes, Valium, morphine, Demerol, and chloral hydrate), which some believe were used in par as a sort of "mind control." If you think most of the people were murdered, you should listen to the audio cassette- Jones had the final moments of many, many people recorded. Somehow, it's been leaked from the FBI and you can listen to the 44 or so minute tape.
The Jonestown Death Tape (FBI No. Q 042) (November 18, 1978) - To Archive.Org page:
http://www.archive.org/details/ptc1978-11-18.flac16
What I find most disturbing is that it sounds like these people were very, very devoted to Jim Jones, who they called "dad." I can hardly listen to the tape without thinking that almost all of the people in that room died. 913 people, man. Just boggles the mind.