For those have seen the film or read the book (SPOILERS)

They were equally responsible. Even though, Perry was a more decent human being, he had emotional problems and pulled the trigger. Hickock was there, he set up the robbery, did nothing to stop Perry and both deserved to swing, even though I felt bad about Perry's childhood.

I think it’s almost natural for us to feel more for Perry than Dick, as he is clearly a more 'interesting' character. By this I mean he is much more complicated than Dick and in trying to understand his actions we are more inclined to empathise with him than Dick, who comes across as a common or garden bastard whose actions are easily understood. In both the book and the film it is strongly suggested that it is the combination of Perry and Dick that is responsible and therefore I think it is almost impossible to say one is guiltier than the other.

During the murder scenes (which are unbelievably horrific and compelling) I think that Dick is intent on raping Nancy, but doubt that he has the constitution to live up to his boasts of leaving no witnesses. It is Perry who stops Dick from raping her. In the book in particular, Perry suggests that Dick would never have had the guts to kill the Clutters, especially in the way Perry did; in cold blood with them tied up and defenceless. Also Perry's motivation in killing them seems to stem from an antipathy to Dick rather than to the Clutters, in both the book and film he is obviously disgusted by Dick and holds him in contempt, shown when he says that he 'cannot stand people who can't control themselves.' However he also has a lot of affection for him, which is one of the things that make him a complex and therefore interesting character.

Without the combination of the two the murders would likely have never happened and, although I certainly understand why people can feel more antipathy to Dick who, unlike Perry has no mitigating psychological circumstances, I think both must be equally condemned

The beauty of this film is that it shows that characters who you engage with, and can even be said to be likeable, can be capable of horrific actions. This is not a film that excuses murderers by humanising them but one that condemns the murderers’ inhumanity by showing the extent of their humanity.

I have read the book several times and I have a different opinion of Dick that I have never seen really explored. If you recall, it is explained that he was in a horrible accident. It caused head injuries and disfigured his face. Capote notes that his family remarks that Dick seems to have "changed" after the accident. Most of his criminal activity seems to have happened after this. While I don't think he was ever a great person, he seems to have lost his ability to control his baser impulses ie bad checks, sexual...
I have read of studies where they have explored criminal behavior after head trauma and I've always suspected that Dick would not have turned out he way he did if not for the accident.
Not that it excuses either of them for what they did, but I think Perry's abusive upbringing and Dick's head trauma explains somewhat how they could commit these murders.

Without Dick, the murders never happen. Without Perry, the murders probably still happen, but Dick still may have not been the triggerman.

In my reading of the book, I found that although he was portrayed more sympathetically than Hancock (many say Capote had a big crush on him), I can find no redeeming social value in either of them. Hancock was basically a coward, and a con man, who (as is stated in the book) was only brave when he had power over somebody. He was also a child molester. Smith knew that Hancock's actions were despicable, but was too weak, and too attracted to Hancock's "masculinity" to resist murdering those people. Although he stopped (according to him) Hancock from raping Nancy Clutter, he had no problem blowing the heads off four innocent people. I have heard (and known) people with much worse childhood's than Smith, who grew up to be decent adults, not mass murderers.

"Richard's in good hands, Robin. The best in England."

I think that Floyd Wells could have had no way of knowing what Hickock's plans were. He probably had some remorse about what happened to the Clutters, becuase he knew and liked them, but he still took some time to notify the prison authorities about what he knew becuase he was worried about being considered a stool pigeon among his fellow inmates.

Another poster brought up the fact that Hickock was in a car accident that changed his personality somehow. I do recall the book mentioning Hickock's accident, but the movie does not go into that. Both the book and the film say that Hickock's family was confused and surpirsed at Dick's actions before and after the murders, and don't seem to think the accident was the root of the problem. The accident may partially explain but certainly not excuse Dick's behavior, just as Perry's wretched childhood does not excuse his actions.

I think Perry was a time bomb waiting to go off, and that Dick knew and and exploited that, therefore my opinion that Dick was more responsible. I agree with Silentobserver that something clicked in Perry's head and that the Clutters paid the price for his rage. As Perry said upon being interrogated "I thought Mr. Clutter was a very nice gentleman, right up until I cut his throat." Maybe Perry, just for a few homicdal moments, saw Mr. Clutter as his own father, who he alternately loved and despised.

Oh yes, I agree that Floyd Wells should have some responsibility, too.
I have read a book few years ago, so correct me if I don't remember it exactly, but as far as I remember, Dick told him that he will rob and murder Clutters, but Floyd still had no problem to tell the <false> story that there is cash in the house, giving him the plan of the house, etc.

Interesting question. Without Hickock there would have been no crime that particular night. But sooner or later, Perry Smith would have killed someone in a blind rage, just as he did the Clutters. Many people have even worse childhoods than Perry and do not become murderers. Perry chose to go the way he did. He could have worked hard to better himself, made different choices, but he didn't. Hickock was all talk and no action. Perry was a walking time bomb. I don't think he was ever remorseful for what he did, just sorry he had gotten caught.