The Gremlin's Grampa — Robert L. Fish

A quartet of killings threaten to derail Lieutenant James Reardon's relationship -- and end his life

Jan has been dating James Reardon long enough to know that she wants to be with him forever, but she will not marry him as long as he's a cop. She has spent too many nights lying awake, afraid that this will be the case that gets him killed, and she cannot make that her whole life. But she and Reardon both know that death is the only thing that could make him take off his badge -- and for this hard-boiled San Francisco detective, death may come sooner than he thinks.
 
It starts with a stabbing in the Embarcadero. A particularly sleazy bartender has gotten knifed in the gut, and he is dead before the cops arrive. Three more killings follow, and each time the victim is one of the city's worst criminals. Is it a vendetta, or a vigilante? Reardon will risk his life to find out.

The Gremlin's Grampa is the 2nd book in the Lieutenant Reardon Mysteries, but you may enjoy reading the series in any order.