Originally Posted by Fetish Gimp

Another way could go is this:
Walt goes against Uncle and his cronies, kills them all using SCIENCE! and the big-ass gun. Finds Jesse alive, learns that he's been kept hostage to keep making meth.

Walt makes Jesse realize that as long as he's alive Brock is in danger, Jesse lets himself be killed in order to save Brock.

Walt goes to see his family one last time and finds them all dead. Skyler, Walter Jr, the baby.

The Mexican cartel has finally enacted their revenge, having never forgiven Walter for stepping into their territory. Walt then takes the ricin.