Jo: For years, Leonard Brooks has suffered from sudden impulse control disorder, compelling him to start fires, but he did it to cause property damage, never to cause bodily harm or to take a life.
Mac: So, why does his M.O. suddenly change? And in such an extreme fashion?
Jo: Because, clearly, the relief that he got from starting fires no longer had any value.
Mac: So torture and murder is his new drug of choice.
Jo: And when his emotional need shifted, he needed to find another means to satisfy it.