Popular psychology has a number of tidbits on what can reveal your true self. Over at Whitley Strieber's Unknown Country I have now discovered that I have high self-esteem because I like murder mysteries with a surprise ending. Not that I feel smart when I read mysteries (I never solve them), but that's OK. The bit about why people enjoy mysteries versus suspense stories fits for me, too. I like a rich tapestry of possible culprits, and the best mystery writers offer up a number of logical suspects with ditto motives and opportunities.
I have never thought that no human being is capable of killing. What makes a murder mystery interesting is the exploration of what it would take to make an otherwise intelligent, moral person take such a drastic step (murder mysteries involving a psychopath are no fun). Agatha Christie was good at pointing out that greed, jealousy, and revenge were the usual reasons. It's never more noble than that. But if they're good at covering their tracks, you need a good sleuth to catch them. Which reminds me: I have two P.D. James books to read. I enjoy her sleuth, Adam Dalgliesh.