I think, because there were so many knife wounds, the police assumed that the little boy had been tortured. Although the report was that the child had been abducted, Eddie said that the child had in fact disturbed him while he was self-harming. It seems that Eddie was the victim of extreme parental neglect and possibly others forms of abuse and, on being interrupted and questioned by the little boy, all his anger and frustration came out in the violent attack.
Of course, some people will see the plot as distasteful for a number of reasons, including its similarity to real life crimes and its graphic descriptions. My own opinion was that it raised some important issues. Unlike many dramas these days, it was not cliched and did not use acts of extreme violence to entertain people or reinforce populist views but to make them think.