Hell Bent by Devon Monk

 So, it's been a few years since the final novel in the Allie Beckstrom series, and we're switching protagonists to Shame, the Death magic user whose soul complement is Terric, and the pair has been serving as the co-heads of the Authority after magic was neutered by the events in Magic for a Price.  Only soul complement pairs can now "break" magic and make it perform as powerfully as it used to, and most of the Authority members have quietly faded into the woodwork.

But someone has been committing murder by magic, and it becomes Shame and Terric's responsibility to find the culprit, even as they are removed from their position and replaced by a chairman more suited for quiet times. It has been rumored that some shadowy branch of the federal government is trying to figure out how to use soul complements as weapons, and so the known pairs have mostly fled or gone into hiding. Only Allie and Zayvion, Shame and Terric remain in Portland, and stubbornly refuse to leave their home.

I'm not generally enamored of the self-pitying, anti-hero, which Shame appears to be, but his attitude almost approximates self-hatred, or at least hatred of the monster he thinks he has become, constantly fighting the temptation to use Death magic to siphon the life out of everyone and every animate thing he encounters. For the most part, he's coping in the traditional manner - boozing, smoking and avoiding human contact, aside from the ghost, Eleanor, who has haunted him for years now. When a government assassin gets him in her sights, he rapidly finds himself more engaged emotionally than he prefers to be, and she finds ways to get him properly motivated to pursue the killer.

This novel seems like a pretty good setup for a new series, continuing an old idea. Monk always writes an entertaining tale.