Presumably this is Boddekker's Demons by Joe Clifford Faust. The publication details are on the Internet Speculative Fiction Database here.
Boddekker's not just a faceless copywriter anymore, grinding out ads for VR simulators and mood/attitude orienters. Now he's the "five-hundred-pound gorilla" of the Pembroke Hall agency, lumbering toward fame, fortune, and a fabled house in Princeton with shapely Honniker In Accounting. All thanks to Ferman's Devils, the four New York City street thugs whose record-breaking (and bone-breaking) commercial has won them billions of fans and made gangster chic the latest rage.
Yet Boddekker's new proteges are on the fast track to hell. First they assault a famous talk show host. Then they murder a former child celebrity in cold blood. And the more outrageous their actions, the more popular they become. So when another murder occurs, this time just too close to home, Boddekker knows he must find a way to take the Devils down. He made them and he can break them...if they don't break him first.
The ISFDB is an immensely valuable resource for finding books when you half remember the title. I did this advanced search for all novels with "demon" in the title and this book leaped out at me.