In the fable "The Scorpion and the Frog," a scorpion asks a frog to carry him across a river. The frog is reluctant - surely, to deal with a scorpion will bring certain death. But the scorpion is adamant that this train of thought is nonsense - that if he stung the frog, they would both drown, and die. Midway across the river, the scorpion stings the frog. The frog asks why the scorpion has condemned them both to death with its sting, only to be met with the obvious - "I am a scorpion."
Fiction demonstrates many concepts, but none so aptly as the fact that some characters are designed by nature to be purely evil and serve that purpose exclusively. To put it another way, some people are scorpions.
Dishonored is not a game about making easy, simple choices. Rather, reflective of real life, the game soon demonstrates that you are interacting with the fate of an entire metropolis. The stones you're throwing in its pool are going to rock someone's boat, regardless of how good your aim is.
Warning: Spoilers ahead.