what's so difficult about this question
You have to weigh 1 life against many. Which raises the question of just how do you value a life?
Also, the question is usually phrased to ask which is the more moral choice.
If you do nothing a large group of people die. But if you do something, while you save a lot of lives, you choose to kill a person.
Is it amoral to let people die if you have no responsibility? Is it even less moral to choose to kill an innocent man?