Ender's Game (2013)

Answer: For the same reason when you are playing a video game you will run people over, or shoot them without figuring out who they are. He thought it was the equivalent of a video game, i.e. not real, so he would potentially do things he wouldn't do if he thought it was real. This is the very reason the power structure put them into a real operation while leading them to think it was a simulation, so they would be likely to act when faced with a moral dilemma.

jimba