Drive-by (Season Three, Episode Twelve: "Half Measures")
We never saw it coming. Neither did they. After an exquisitely tense build-up in which a tweaked-out Jesse prepares to confront the dealers who murdered his new girlfriend's kid brother, Walt white-knights from out of nowhere, wielding his SUV as a WMD. Appearing from offscreen to run the dealers over, he then gets out and shoots the struggling survivor to death at point-blank range. Walt still really cared about Jesse at this point, enough to risk not just his life to prevent the kid from becoming a murderer, but his own soul as well. He killed more than a couple of asshole child murderers – he killed the part of himself who once wrote the words "MURDER IS WRONG" on a note to self. This wasn't just a murder. It was a murder-suicide.