First of all, the bedrooms were pretty close to each other so it is very possible to shoot the parents, both brothers and one sister in a very short amount of time. Also the picture of the "7th body" was not one of the DeFeo family members. Ric Osuna claims that the picture of this body was on the crime scene pics but this is the same man who took Geraldine Gates' word on everything DeFeo. Lots and lots of contradictions in his book The Night the DeFeo's Died.
Dawn did not have gunpowder on her hands. If you have seen the crime scene pics, she looks like she was in a very natural sleeping position. There are claims that there were unburnt particles on her nightgown but noone knows for sure if these are on the front or back of her nightgown.
George and Kathy Lutz didn't make that much money off of the book. Plus they were involved in a few lawsuits and most of the money they did make went for attorney fees, etc. Also Jay Anson, the author of the book, used literary license and even though the book was written in diary fashion, the events he claimed happened on a certain day didn't necessarily happen when he wrote they did. Anson had been given some tapes made by the Lutzes and, along with his own research, that is what was used for the book. Alot of things that Anson claimed and what the movie claimed was NOT what the Lutzes claimed. As for the 2005 remake, that was a total joke.
Something else to consider. George Lutz didn't care if anyone believed him or not. He just wanted to get their story out there so people would be aware that these things do exist.
A good board to visit is http://www.amityvillefaq.com/truthboard/