I've been writing out the plot to a game, and it's going for a particular message. In order for this message to be conveyed, I need to get the player to commit murder without thinking about it. This much isn't really hard. But what I need most, and where I hit a stumbling block, is making them murder somebody in a time and place where it's clear this is wrong, but I can't actually force them or the message will fall flat. This needs to be avoidable, and all of these murders need to be avoidable, but the player has to fail to realise this the first time through and kill people they shouldn't have. More specifically, the game has the player chasing an assassin who killed their first character (a small boy) and a girl he's with in cold blood. They break a lot of laws along the way, and kill plenty of people who are in their way but don't deserve it. At the end, they finally track down the man's family trying to get them to give up the man's location.
At this point, the player is directed into speaking directly to the man's wife. A lot has happened to make them and their team angry along the way, and this is where I don't really have a plan anymore. I need the player to try and beat the information out of her, and kill her in the process. In front of her children. After this the game shows this scene (followed by every other murder right back to the first) from the eyes of their victims, then shows the murder of those children at the beginning from the killer's perspective. This is made to show how similar their actions were, and the message should be clear.
The issue is I need to make sure the player's conscience doesn't kick in and stop them the first time they play through, but still allow them to avoid all these killings in later playthroughs without changing anything in the game itself, so they can realise that everything they did could have been avoided.