Walter White, despite spending most of his life working as an unappreciated high school chemistry teacher, is probably one of the most terrifying individuals on modern television. He's not physically imposing, or particularly athletic -- it's not like this guy is going to karate chop you or something. But he's scary because he has no limits. There is no point, in all of his dealings with the seedy underbelly of the drug world, where he thinks, "OK, maybe I'm going too far." There is no "too far" for Walter White...he'll do what it takes to get his money, and God help you if you stand in his way. When Walter first gets his cancer diagnosis and makes the decision to start making crystal meth, he's kind of a joke compared to all the hardcore dealers that he has to interact with. He's a fastidious chemist, determined to make the best product he can. It's almost like if he runs his operation meticulously with every detail taken care of, he can convince himself that he's actually running a legitimate business. But by the end of the show, he's no longer a good guy forced into the drug world by circumstances -- after all, he is The One Who Knocks.