SPOILER: The zombies were actually aliens who came to earth to battle the evil race of paracite called "shwaag". But their minds were erased during the battle in the 4th movie. Causing them to go all brain dead on their hosts as seen in part V. As for the first trilogy, the original undead were controlled by the "shwaag". As for this movie some of the good aliens are starting to regain memory, causing the zombies infected with them to go good and withs to the human side. How ever the blood thirst still remains causing them to sometimes breakedown and kill their team mates. And now we have to wait till the part XVIII-3 comes out to see the battle with the "Shwaag"-colossal-zombie-overlord with a flamer gun hammer of ARGAST" for the faith of earth.
__________________________________________________________ Still a better movie idea then twillight