When one considers the several potential suspects in this murder of an entire family, Perry and Dick stand out as the most likely. They had already murdered an entire family for no sane reasons. They traveled all across the country and were in the close vicinity of the Walker home at the time of the murders.
They really needed no "reason" or "motive" for killing the Walker family, and anything they might have taken from the scene could only be rationalized by realizing how crazy they were. The movie "In Cold Blood", based on Truman Capote's book by the same name, is very good in portraying these two killers, but it probably does not go all the way in showing their depravity.
Scott Wilson (who played Hershel Greene in the Walking Dead) was a young 24-year-old actor chosen to portray Dick Hickock in the movie. He, possibly more than anyone else, really studied Hickock and tried to get into his mind. Here is an interesting interview in which Wilson discusses the making of the movie, and his thoughts on the murders:
An interview with the actor who portrayed Dick Hickock in the 1967 film.
paleymatters.org