When, from a design standpoint, does violence "work"?
When it's necessary to the narrative. Some of the moments in games that are most widely remembered and appreciated involve acts of violence, like the plot climax of BioShock or the end of Metal Gear Solid 3 — cases where the player is asked or forced to execute a death in a way that enhances the story.
When an act of violence is a crucial part of a game's story (assuming the story's well-established), the player naturally takes ownership of the action and its implications. That sense of agency is supposed to be one of the strengths of interactive entertainment, so it makes sense to be judicious with it.
Giving players the opportunity to perform any action in a very specific and intentional context virtually requires players to think about what they're doing and be engaged.