"Take ye heed, watch and pray: for ye know not when the time is."(30)
"a person or persons unknown." (77)
"[Now free to do as he pleased,] the murderer killed them one by one, [each time carefully collecting the discharged shell.] When he had finished, he turned out all the lights and left." (82)
The first two phrases are every ironic. The first one, it's true they never knew when the time has come for you to part from this world. Certainly, the Clutters didn't expect that it'll occur soon, not on Thanksgiving week at least or anytime soon. The second one, technically, to the investigators the murderers are 'unknown'. But I think on Capote's perspective the murderers are much more than 'unknown', that they also have an identity, a past -- no different from us nor the Clutters. Lastly, I feel like the way Dewey describes the murderer is just doing his chores. Paraphrasing it to "he planted them [seeds] one by one" and when "he had finished, he watered them and left." Like murderers are immoral people, and it's completely normal that they murder, they show no mercy because they don't care.
Clearly, Capote's style in writing came from a different era, used simple words, excellent description of the characters and places. His writing grabs you from your chair to exact scenes that happened, as if you're a ghost that follows each characters, you feel the emotions, the shivers -- in cold blood.