Dying Light would like to remind everybody that zombies are/were people too.
Zombie games, as saturated as the genre may be, do require a bit of finesse to do right. On one hand, being a talented slayer of zombies feels awesome, but on the other hand it gets depressing quickly if you stop to think about the state of humanity in these settings. Since Dying Light's fast-paced parkour gameplay is probably going to be a lot more of the former than the latter, the developers are making sure to tug some heartstrings with their pre-release trailers. The latest of those trailers, "Humanity," takes a moment to remember who those zombies used to be, before they became walking experience points.
Alright, so it's not the cheeriest subject. It does tie in with Dying Light's emphasis on the morality of post-outbreak living, though: will you buy into the "kill or be killed" mentality, or will you risk your life to preserve your humanity? Expect to see plenty of choices like that in between the game's desperate chases and improvised-weapon melee brawls.
The story behind the Dying Light outbreak is pretty familiar: the infection took us by surprise, there was no time to stop it before civilization broke apart, the afflicted roam the streets craving brains, so on and so forth. The game itself takes place in the quarantined city of Harran, so it isn't quite clear how the world outside the city is doing - but inside the walls with you are a handful of survivors, an army of zombies, and a society's worth of supplies to loot, so that's probably more interesting to the player.