A big boy did it and ran away

The first book in the Angelique De Xavier series, from author Christopher Brookmyre. Back when they were students, just like everybody else, Ray Ash and Simon Darcourt had dreams about what they'd do when they grew up. In both their cases, it was to be rock stars. Fifteen years later, their mid-thirties are bearing down fast, and just like everybody else, they're having to accept the less glamorous hands reality has dealt them.
Nervous new father Ray takes refuge from his responsibilities by living a virtual existence in online games. People say he needs to grow up, but everybody has to find their own way of coping. For some it's affairs, for others it's the bottle, and for Simon it's serial murder, mass slaughter and professional assassination.