In Season 2, Jesse falls in love with his neighbor, Jane, with whom he begins doing heroin. Walt, who’s furious that a high Jesse jeopardizes their first big drug deal with Gus Fring, refuses to give Jesse his share of the money until he can prove he’s off heroin. Jane then blackmails Walter into handing over the cash.
Later on, Walter meets Donald Margolis, Jane’s father, at a bar, without knowing who he is. In one of the show’s most underrated scenes, Walt talks about Jesse and Donald talks about Jane, with neither of the two having any clue as to the connection. This inspires Walt to visit Jesse and make amends. While there, he tries to shake Jesse out of his sleep and ends up accidentally rolling Jane onto her back. Overdosing, she vomits and begins to choke. Walt rushes to help, but, realizing she’s a threat who knows too much, lets her die instead. In the aftermath, Jesse is shattered and Donald, an air traffic controller reeling from grief, drops the ball at work and causes two passenger jets to collide in mid-air.
Of course, that plane crash is not a reasonably foreseeable consequence of Walt’s actions. But even without that, devastating Jesse by letting Jane die, a crucial moment in Walt’s transformation to Heisenberg, is the single, most chillingly evil thing he ever did. And that’s saying quite a lot.