Sacrificing a few for the majority (or just saving a few)

So you are now in a situation where you have a decision to make, you either sacrifice a few or risk many more dying, what do you do? Do you attempt to save everyone? Or do you let a small amount of people die so the others can survive. Here are a couple of scenarios in case you were having any trouble thinking of some.
[li]Rather than a few, you’re just sacrificing one. As an example in order to kill the invading Aliens who will either take 10% of the worlds children or kill everyone, you have to use a kid as a broadcasting device which would kill him (taken from Torchwood)[/li][li]Part of your military tactic is to send in a squadron purely as a distraction, knowing that it will kill them. But you can see no other way you can win the war.[/li][/ol]

And what about the converse? It is only in your power to save a handful of people, will you try to save everyone? will you save those you love? or will you save those who have more value (lets say these are gonna be the only survivors of an extinction event will you save your loved ones or say a physicist, an engineer, a doctor etc)?

Just hypothetical (and because I’m still procrastinating)