I suppose, in the end, pretty much all the characters were either dull or irredeemably unethical. I don't know why they had to drag even Wallace Shawn (played by Bob Balaban) through the mud: his own children protested that the New Yorker editor never went out to Kansas to witness the execution or anything as intimately involved as that.
One thing bothered me a lot about the screenplay: how come the other guy, Dick Hickock -- the one who didn't end up a kind of weird crush/alter ego/pawn for Truman Capote, also got executed? According to the film, he didn't murder anyone. Perry Smith killed the entire family. True, Hickock said, "We shouldn't leave any witnesses!" but that's not the same as slashing someone's throat or shooting them in the head.
I know this wasn't meant to be a courtroom drama -- it was intended to be an examination of the way that an author got drawn into a dangerous subject, and what it did to him. But still, I thought a scene that let us understand that there was never any clear evidence which of the men actually murdered the Clutter family (Perry Smith confessed to all four, but I guess the jury didn't buy it) would have helped. As it was, I was left with a much more negative opinion of the whole question of justice, and Truman Capote's interference with it, than was perhaps warranted.
Oh well. That's Hollywood.