You betrayed the Prepper working with the Ravens of Dee and ight. Now mad at you, you try to convince him you're on his side.
It has six months since the outbreak in London. However, despite attempts of containment, the plague managed to reach the shores of the U.S. Eastern Coast, devastating almost everything east of the Mississippi. The military's final plan is to wipe out whatever remains from Boston to Miami.
My own take on the ending for ZombiU. Warning: ending contains intense spoilers and violence. Read at your own risk.