The first reason is realism. There have already been many books written about idealistic lawyers racing against time to defend their wrongly accused clients. On the other side of the legal fence, there have also been many books about idealistic prosecutors on a crusade to put all the bad guys behind bars. But, the fact is that after a few years spent fighting in court idealism is often replaced by cynicism. Defense attorneys stop caring about their clients and start caring only about winning to feed their own egos, or to get paid. And prosecutors, with time, begin to feel under-paid, unappreciated and over-worked, and so they spend their days just trying to get through their huge caseloads without having a breakdown.
So, in The Guilty, the main character is a defense lawyer who doesn't care in the least about his client. In his particular case, he only cares about winning and, as can happen when anyone begins to focus solely on satisfying his own needs, the potential consequences of his actions are ignored.
I think this is a realistic look at what can happened to a defense attorney. Or am I being too cynical?