In today's legal system, we would try Perry Smith with second degree murder and Dick Hickock as an accessory to murder. This assessment alone disproves Truman Capote's insinuation that Dick was a heartless, cold-blooded-killer and Perry was a helpless, malleable, but angry little man.
A psychopath is defined as a person with an antisocial personality disorder, manifested in aggressive, perverted, criminal, or amoral behavior without empathy or remorse. Psychopaths meet legal and psychiatric standards for sanity as they understand the rules of society and the conventional reasonings of right and wrong. They are capable of controlling their behavior, and are aware of the potential consequences of their actions. The problem is that this knowledge frequently fails to deter them from their antisocial behavior. Despite this diagnosis, Capote used an almost caring tone when referring to Perry throughout the novel. It was pitiful at times, but mostly of a sympathetic nature, a softness unnecessarily deserved. While when describing Dick's character and goals, Capote used a shallow and casual tone, as if disregarding him as simple minded and selfish.
Capote appeals to pathos as he points out that Perry didn't consider raping Nancy, like Dick had, and came from a difficult childhood, explaining it in tremendous detail. This influences the audience's opinion of Perry, that there is a possibility that despite Perry's lack of consciousness throughout the planning of burglary (and eventual unplanned killing), at least he dissuaded Dick from a torture worse than death. Society believes that a quick shot to the head is merciful compared to a grueling rape, thus making Dick a heartless, calculating, cold-blooded-killer. It also gives reason to treat Perry with less severity and criticism as we learn of his atrocious childhood. .
There are many reasonings for Capote's bias towards Perry as Capote himself was abused as a child and he had a certain fascination with Perry for this reason or romantically.