Thankless in Death by J. D. Robb

In this thriller in the #1 bestselling In Death series, Lieutenant Eve Dallas discovers that deep hatred can seethe within the closest relationships when she hunts down a man who murdered his own family.

Lieutenant Eve Dallas has plenty to be grateful for, especially Roarke’s big Irish family, which is a joyful improvement on her own dark childhood.
Other couples aren’t as lucky. The Reinholds, for example, are lying in their home stabbed and bludgeoned almost beyond recognition. Those who knew them are stunned—and heartbroken by the evidence that they were killed by their own son.
Turns out that twenty-six-year-old Jerry is not only capable of brutality but has taking a liking to it. With the money he’s stolen from his parents and a long list of grievances, he intends to finally make his mark on the world. Eve and her team already know the who, how, and why of this murder. What they need to pinpoint is where Jerry’s going to strike next.