First post, here comes the great wall of text...

In the year 2104 in an attempt to establish world peace once and for all after a third world war comes close to wiping out humanity, 31 of the largest nations on earth agree to scrap their individual governmental systems and form a collective public. From that point on all people are equal in all ways, with decisions being made by consensus of the general public and everything being "owned" by the collective of humankind rather than an individual person or the government. Any anomalies in the system were quickly removed by a force of "peacekeepers" known as overwatch police. The society functions in relative peace until 2117 when a small group of resistance fighters form an organised rebellion and attempt to reinstate ownership of their belongings. The true destructive nature of mankind rears its head once again, and 13 years of repressed violence erupts in an all out war between commoners and peacekeeper officials. The situation quickly escalates, with the resistance movement growing larger by the hour and major logistical facilities being liberated. The situation comes to a climax when the resistance march on the overwatch bastion. Faced with certain annihilation, they resort to using the ultimate 3rd war bioweapon that was secretly preserved in anticipation of this eventuality by the old government. The bioweapon will consume human organic matter within seconds and use it to reproduce. The weapon is released into the air, and all of mankind save for the few thousand overwatch chosen to survive is wiped out.

In the game, you play as a high ranking overwatch official. For the first part, you are retreating back to the Bastion. At the point when you reach it, one of the characters you fight with throughout the first part starts a consensus to use the old weapon. Your character agrees at first, but as he sees the entire of civilization being wiped out in front of his eyes, he starts to realize the ramifications of his actions.
At this point you chose to go rogue and the game becomes open world survival in a world were people are moreorless non existent.