Immersive sims allow the player to express themselves within their systems. If I want to be a cool guy who upgrades his pistol and shoots dudes in the head, I can do that. If I wanna be a clever ninja who cloaks and jumps really high, I can do that. If I wanna build my character so he only cares about throwing vending machines at people, I can do that. I can, to some extent, customize my experience and play with the systems in my own personal way.
Many immersive sims allow the player to play through the entirety of their stories without killing a single person. Deus Ex Human Revolution is one such immersive sim.
However, DX:HR also allows you to go the opposite direction. If you so wish, you can kill almost every single character in the entire game. The only exceptions are plot-critical characters who live in Sarif industries, where the player’s weapons and combat skills are removed.
Inspired by my friend Justin (who has, as far back as I can remember, reveled in massacring as many innocent, virtual people as possible in every game he picks up), I tried to complete two different DX:HR playthroughs: one where I killed nobody, and one where I killed everybody.