2. Walter Will Kill Himself
If the show creators want to end the series by making Walter a sympathetic character, it is fairly likely that we will see Walter so ashamed of his actions that he thinks the only noble thing to do now would be to kill himself. That said, although I once believed the show was trying to make Walter a sympathetic character despite doing surprisingly horrible things I now don't think the writers want Walter to be sympathetic at all. In fact, Walter's journey seems to be all about starting off as a kind of good die, and ending the series as one of the worst human beings imaginable. But if Walter's world starts to fall apart like is suggested in the season 5 opener, and his family all find out about him and are disgusted and end up leaving him then maybe this will give Walter the shock he needs to really evaluate what has become of his life and decide he simply can't take it anymore. We know that part of Walter actually wants to die... the only reason he started cooking meth in the first place was because he believed he had mere months to live, and when he found out he no longer had cancer and wasn't going to die, he was actually pretty pissed about it. It is also fairly possible that another horrible act will push him over the edge. We saw that when he allowed Jesse's girlfriend to die, he found this pretty difficult to take. When all those people died in the plane crash that was partly his fault, he basically had a mental breakdown of sorts. When he almost killed that kid it didn't seem to effect him as much as times in the past, but he still wasn't totally comfortable with himself or his actions. What if for example he kills Jesse? Will Walter be able to cope with the killing of a man whose become as important to him as a son? What if Walter kills Hank? Will he be able to live with himself? What is Walter causes his wifes death somehow, or perhaps even his whole family die? He surely couldn't live with that could he? It is also entirely possible that perhaps some of his previous colleagues , the Neo Nazi's or some old friends of Gus's will for some reason want revenge, and up taking out Walter's loved ones which leads Walter to buying the machine gun in order to take revenge on those who have destroyed his life. I couldn't really imagine Walter shooting at the DEA with a machine gun, but I could absolutely imagine Walter taking revenge on drug dealers or henchmen if they for example they kill his wife Skylar. This seems like a likely use for the machine gun.