Betrayers — Bill Pronzini

We're told that there are seven deadly sins; not on the list is the deadliest of them all: Betrayal. For each of the detectives at the agency, a betrayal--personal, against a child, against the elderly--becomes not only the driving force behind an investigation, but the source of the kind of resolve that cannot be derailed by threats of any kind.
Tamara's case began as something personal but explodes as her investigation of her former lover Lucas Zeller leads to a scam bilking charities in the name of helping the homeless and indigent. For Nameless, with a case he doesn't want but can't turn down, trying to find out who is gaslighting an old woman only exposes the ugly side of family. When he goes home, tired and annoyed, he discovers that his adopted daughter, Emily, has a secret of her own. Runyon has a different difficulty: his case of a bailjumper with some bad family ties is easy enough as these things go, but he's being confronted by a demon that is going to try to force him into a betrayal....

Three people who care, three people devoted to helping others trying to help themselves, three people finding themselves in a world of hurt because of the betrayers.