Lazybones - Mark Billingham

Someone - a woman or somebody pretending to be a woman, is writing to convicted rapists in prison, befriending them and then brutally killing them when they are released.
DI Tom Thorne must discover the link between these killings and a murder/suicide that took place twenty-five years before; a tragedy to which the only witnesses were two small children, now adults and nowhere to be found...

How can you escape a past that will do a lot more than just catch up with you?
And how can Thorne catch a killer, when he doesn't really care about the victims?