In many video games, you must kill stuff all the time, and quite frequently. Killing becomes the environment. It is so pervasive that killing becomes the context for something else -- clicking on a soldier's face rapidly enough to demonstrate mouse dexterity, or chaining together different button presses to make combos, or optimizing your stats to make a big number even bigger, or carefully managing various bars before they deplete. The killing is rarely about the killing. (Which makes you wonder why we need to wrap it in the narrative of killing.)
During an Elder Scrolls game, you will likely kill thousands of things. However, all of those killings are sanctioned by the NPCs in the game: you are killing monsters outside of cities and villages. Their deaths don't matter -- more will respawn to take their place, or maybe the game will delete them to free-up memory when you wander away far enough. They exist only to be killed. They are domesticated and farmed.
The Dark Brotherhood questlines in Elder Scrolls games, then, are one of the few instances in games that really focus on killing as killing. Specifically, it frames murder as a deeply anti-societal, anti-social, transgressive act, and explores the philosophy required to justify it. At it's best, it's also deeply systemic.
