Why did Dick cut Mr. Clutters throat in the book "In Cold Blood"?

Perry claims to have liked Mr. Clutter, as well as the other members of the family; this at first seems ironic, but in fact it says a great deal about Perry’s motive for killing the Clutters. The murders, it seems, were not inspired by a literal hatred of this specific family, but by misdirected frustration and resentment that finds a symbolic object in the Clutters and the values that they represent. The family is unlucky enough to be on the receiving end of this fury, but they are by no means its source.

“I didn’t want to harm the man. I thought he was a very nice gentleman. Soft-spoken. I thought so right up to the moment I cut his throat.” Perry, pg. 244