My Horrible ending theory!
Walt's special meth has been seen in the walking dead so my theory is that Walter's blue meth unlocks a DNA Strain that just so happens to raise the dead meaning everyone directly or indirectly that ever handled or used it become "infected" now back to walter and how this works for him he has a massive shootout at that place that jesse went to after jane died with all the junkies in it.
Walt battles against the DEA and then starts breaking down in a coughing fit as the cancer comes back and starts to take his life (because he worked around all the chemicals it worsened his conditon) as undead junkies start surrounding him as walt sees what his meth has done to the world as he dies and becomes the first infected (he is the first because he was the first to be killed by zombies)
And then all the zombies go outside and infect the confused DEA Who then are confronted by cops and the like who don't want to kill the other law enforcement whom they know also get infected who then infect the general populace and since anyone who directly or even indirectly (indirectly being where a person who has handled it comes into contact with another person who then spreads it to others resulting in a massive chain as infected have traveled infecting pilots and everyone else they meet during there travels. resulting in the world being infected by walt leading to the events of the walking dead.
This theory is arguably the best one and most plausible
Oh and By the way Im just fucking around.