Disturbed

Charlie, still feeling guilty about Don's injury, focuses on finding a frighteningly intelligent serial killer who is meticulous about eliminating all witnesses to his crimes, thereby murdering unobserved and undetected.

Title

 * There is a dual sense of the inhumane behavior of the killer and the perturbation of Charlie's worldview brought about by his awareness of Don's mortality and his own fallibility.

Trivia

 * This was, as the opening numbers sequence mentions, the one hundredth episode of the series.
 * The episode addresses what might be termed "Superman syndrome." In the 1960s and 1970s, comic book fans increasingly wrote to the mail pages of their favorite titles asking why the most powerful heroes, like Kal-El, couldn't just end all the real world's problems. Specifically, why Superman didn't fly at a massively supersonic speed and grab up all the world's nuclear missiles, then cast them into deep space. Both Don and Charlie have to, in a brutally personal sense, come to terms with their limitations as, at the end of the day, human beings, mortal and vulnerable.
 * "Some people drink, some gamble - I analyze data." This may be the clearest piece of self-reflection on Charlie's part in the whole of "Numb3rs" as a show.