But if Greg pulls the lever, he is deliberately choosing to bring death to this gang of five. If he leaves it alone, it will not be him who causes deaths among the forty.

This seems to be the real question here: Is it better to intervene to cause the death of a few rather than to let events play out that will cause the death of many?

The decision is agonizing because it's only human not to want to associate yourself with the death of others. Also, in addition to this, we worry about imperfect information, and wonder if our choice might be made worse after we get more accurate information about the situation.

Personally, assuming that the situation is simply 'allow 40 to die' or 'kill 5 instead', the best outcome is to kill the 5. If I were in the group of 5, I would want things to play out in that way. Of course, no one wants such a situation to arise in the first place. However, IMHO not intervening when you can is the same as intervening.

Actually, I think there's an argument to be made that it's only because some people have made such difficult choices in the past that we aren't ruled by tyrants. IMHO there's nothing fundamentally wrong with making difficult choices. I think what most important, is that the choices involve scrutiny and consequence commensurate with the impact of those choices. We should identify where in society these choices are made, and scrutinize the hell out of them.

Here's a way to make the choice even more agonizing: include your child in the 5. If that's the case, I won't intervene, and will face the consequences.