I haven't told you about Perry Smith. 

This book is neither about Clutter's murder nor about Holocomb. It's about Perry. 

Poor dear Perry, mocked Perry, downtrodden be life, dreamy, sensitive, talented, poetic, romantic, lonely, misunderstood Perry, whose alcoholic mother died in his childhood and whose father and two siblings committed suicide. Why the life was such severe to you? 

Perry, living with a constant pain in the legs, lost in the darkness. Kind Perry, who underlaid corpses with cushions, bitchy monster Perry, why why did you kill 'em?

I had one vague thought, but it wasn't formed. My mind struggled. I thought impossible to justify these bastards, these beasts who killed so cruelly innocent family. But it's not as easy as it seems

Perry is not the executioner, but the victim. The victim of the society, the victim of his parents, of the people's indifference and hatred. All are guilty. Little by little, every Perry's torturer was killing Clutters.
Moreover, we all are still killing Clutters by our indifference to each other