Interesting dilemma never a jw.
I wonder what a panel of christian ethicists would make of it. It's always tempting to change the terms of a thought experiment to escape the dilemma but that misses the point doesn't it? Would it be a moral good - or less of a moral ill - to kill humans who, through no fault of their own, posed a threat to a large number of others? Would theists and secularists really approach the problem any differently once we get behind the rhetoric? I'm really not sure.
Objective facts about the consequences are a tool that should be used to help us make good ethical decisions. But that doesn't mean there are always easy answers. Sam Harris points out the difference between ethical answers in theory and in practice. How many birds are there in flight around the world right this moment? The answer does exist but there is no way to reliably access it.
There is a great deal of low-hanging fruit that a secular morality could help us with. Other parts of the moral landscape might be far more challenging to access.