Well, I have read a few books on the subject, some on Jim Jones, others on the Branch Davidians.
Jim Jones was deep into the word at a very young age. Somewhere through the years it got twisted and perhaps he felt as though it gave him some sort of power, made him better than others. Later he had a mentor. Its been a while, so I do not remember the mans name, but this mentor was an obvious fraud, was good at working the people, a great showman, so it seems to me that Jim Jones began with manipulative intentions.
The Branch Davidians have a very long history, far before Vernon Howell (Koresh) was ever born. They began as 7th day Adventists. Yes, the Adventists had/have the prophet Ellen G. White, so their beliefs were a bit skewed from the beggining. In the early 1900's a group of them had a falling out with the Adventist church over some "new revelations." These people formed a compound and became the Branch Davidians. They had several wacky leaders before Koresh came around. I believe these people started off right, and through time got their beliefs all skewed, then with the help of some teachers with their own agenda, they started their downfall.
I had attended a Adventist church for about 5 months, till God showed me there was something was wrong with some of their teachings. I absolutely believe that the people at this church I attended are totally, 100% sincere. Even with their wrong teachings, these people are to this day some of the most dedicated Christians I have ever met.
I believe that if you have two parents who believe false doctrines, they have kids and these kids grow up with these teachings, knowing nothing else. Most of these people are apologetics freaks. They know how to defend their wacky beliefs and this is poured into them from a young age. They are taught that bad people will come around and say Ellen White is false. Actually, this was one of her last prophesies, that the devil would send people to try to refute her visions. They grow up hearing that their beliefs are the only true beliefs, and all others are just satans way of trying to lead them astray.
So, if you are ever trying to witness to one, the hardest ones to get through to are the ones who have been raised in it. Sadly, I believe they are very sincere, just brainwashed.