It was Gale's murder that I was referring to. It is equally valid, though, to say that Victor's death sets the tone for the season. This pair of deaths together really accomplishes a lot. Jesse has blood on his hands. Walt means business. Gus means business.
Listen to the latest Previously On... podcast episode for a good discussion on these two deaths. I just listened to it this morning. The next best thing to having a geeky conversation is listening to other nerds have a geeky conversation.