Truthfully? I say there is no right answer. At least not without narrowing it down to one specific circumstance.
Morally, there are few constants if any. Law serves not the will of what is "Good" but the will of order. And thus formallising such a situation tells more about how you view Law vs Chaos, than it does about how Good or Evil one is.
Naturally, in a situation such as this, saving either would be fine. Saving both in either situation may be preferable if possible (At least for some). And whatever situation you come to, it will probably err on the side of good, so long as you're still attempting to save one.
Of course, walking away and leaving both of them to die is downright cruel. So the least you can do is call a rescue team. So yeah, Evil still factors in, but only when you're doing something you think is wrong on a moral level for your own purpose.
As for whether having the Murderers die, Then there's the simple question: Will their death prevent more death? Will the man walk away and never kill another man, or will they slaughter everything? And is it possible to stop them from killing without killing them?
If your answers are that killing him is literally the only way to stop him from killing. Then there's your answer for that situation.