Jane Margolis (Season 2, Episode 12: "Phoenix")
First things first: If you think Walt didn't kill Jane, the young tattoo artist whose affection for Jesse knocked her straight off the wagon, you're in the wrong reading group. One look at Mr. White's face as he watches Jane choke to death on her own vomit following a post-blackmail bender with Jesse says it all: he knows he's responsible for the loss of her life, and he's making the choice deliberately. Sure, he's got reasons beyond self-interest – he's terrified for his family, and he legitimately believes Jane could cost his young friend Jesse his own life in turn through their shared addiction. But ultimately he's presiding over the end of a life in order to save his own skin. And he knows it.