Verses for the Dead (Agent Pendergast)

It's a pretty good mystery and I like Agent Coldmoon.
But there were two things in the book I didn't like.
The non-spoiler thing is the now standard scene of Pendergast crawling around the crime scene,
picking up invisible things with a tweezers and putting them in test tubes that magically appear out of whatever he is wearing.
We almost never have a later scene in which Pendergast announces that after examining the single hair he found, he knew the killer owned an Albanian Moose Hound or something and that answers some question in the investigation.
So whats the point of the scene? We already know Pendergast is smarter than anybody.
Now the spoilery bit.
The motives of the father serial killer are sort of explained.But I don't get why he needs to kill several people as killing one would have made his point.Then, after botching his first solo kill, the son serial killer starts killing people in a completely different way and for no explained reason.The father taught the son how to do a specific kind of murder but then the son does a completely different murder while using capitalized words like Atone and Action to make it look almost religious.In the end of the book, there is no explanation as to why the son is doing murders.We get a fairly detailed analysis of the father but the sons motivation is missing.