dick blamed the killing on perry. they said it was a robbery gone totally wrong the safe isn't there and then they end up killing the clutter family. perry killed keyon and herb while dick murdered bonnie and nancey they had no motive they came in the house with items to kill the family with but they were only going to kill the family if they got in the way of the robbery and when they came to find the safe was not there they decided to murder the witnesses.
Well if this doesn't help much you can always go to Youtube and watch http://www.youtube.com/watch?v=JkCFb3Csg-4 she has several videos telling you a short summary plus more on the novel. Hope I helped
- queen slacker