yeah - when jesse got andrea killed... I think that became a clear possibility... he ends up stepping in for Brock.
Lydia sends todd at skyler, who barely escapes. Walt gets a trunkful of guns, uses some wit and brains, and kills that crew. Jesse escapes and runs to brock. Walt finds and finishes lydia, then does something for grey matter, says goodbye to family, and checks out. Looks like the cancer is getting him and he's enjoyed his time in the wilderness. It's time for him to end things, its run its course. Time to close off all the mess he has created.