A number of games have path-branching storylines which alter the gameplay content based on the choices your character makes. Typically, a large number of those options will be moral in nature. high roads, low roads, grey vs. grey, and sadistic choices. Not every player takes the high road.
Sometimes it's a matter of wanting to see the entire game: certain missions only open up if you're a totally degenerate jerk. There are others when you're simply curious: so what happens if I go against all common sense and most legal statutes? And every so often, you're just blowing off steam: I could never do this in real life, but I'm going to work out all my stress by doing it here. So you blow through the entire game in an orgy of evil because it has no consequences in the real world plus you just wanted to see what that path ending looked like.
Such games, if successful, do what all strong-selling games tend to do: generate sequels. And since you had fun the first time, you eagerly purchase the II and boot up the game, wondering what the first zoom-in on the rubble you left behind will reveal.
And the city is perfectly intact.
All the civilians are fine.
The power plant you blew up for the fun of it is still there, which explains the lack of irradiation among the common folk.
That giant hole where the subway used to be is now a subway.
You worked very hard to make that hole.
But -- in most sequels, the game designers will start with the idea that everything truly worked out the first time, which is why it can start going south again instead of starting there and drilling through bedrock. And so for the majority of sequels to moral choice adventures, there is simply
http://tvtropes.org/pmwiki/pmwiki.php/Main/NoCanonForTheWicked
but at least now you can blow it all up again, right?
...right?
Warning: not a Real Life trope.