When Tanicia Goodwin did the unthinkable, the natural impulse was to understand how this could have happened. If you can’t blame a woman who is clearly, deeply mentally ill, you have to blame somebody, right? The school officials who didn’t recognize tthe danger at home? The judge who returned Goodwin’s son to her custody two years ago? The lawmakers who didn’t pour more resources into child welfare? But there’s another potential answer, harder to fathom and accept: What if the system works, the rules are followed, the risks are correctly assessed, and something still goes terribly, tragically wrong?