Personally, I'm not convinced that I'd know what I would do. I'm not convinced that you know absolutely that you would in this very narrow situation do either.
The conversation starts as,
a bad guy has your family, and says you must kill one person to secure their release, it's easy to say that you'd kill this person, and provided that you didn't have to go to outrageous measures to kill them then it's easy enough to say that you could do that.
you can justify this, the needs of the many out weight the needs of the few, my family, which lets say has four people in it, are a greater number than the one person.
Then the parameters change, can you plant a bomb to save your families life... then the answer is, maybe yes, maybe no, there is a reasonable chance with a bomb that nobody would die.
2 bombs killed three people, in a crowded area, bombs are statistically poor weapons for killing, they are great for inflicting injury or terror, but not very effective killing weapons. So for this thread we'll say a bomb is likely to kill 1.5 people. Needs of the many etc... plus you're removed from the action, it's not like pulling a trigger, of beating someone to death or strangling someone, you put a bag down, it's an easy action.
Now you're talking about absolutely killing 100 people.
Sure you're going to save 50 people, but 50 more people have to die.
It's no longer a justifiable proposition (using the earlier logic) to say well if someone has to die it should be the fewest amount possible...
a more difficult question would be to pose that question the other way around.
you've got a runaway train loaded with chemicals heading for a city train station, with lets say half a million people either in the station or within 2 blocks that in the event the train crashes will be exposed to lethal doses of whatever material is on the train and will die. alternatively, you can crash the train early. But it'll be into the train carrying 25 people including your family.
or go to the logical extreme,
Your family is taken hostage to a particular country, the only way that you can save them would be to start nuclear Armageddon starting with your launching missiles against your home country.
e.g. you're British, your family are taken hostage to Nepal, where you go to rescue them, the only way to rescue them will be to push a button that launches Russia nuclear missiles at the UK.
this will result in at least the total annihilation of the UK population, (with the exception of everyone who is out of the country). and likely cause a country attack by an allied nation against Russia causing huge amounts of decimation there too...
so at least 60million Britons die, plus roughly the same amount of Russians, to save your family, (are we talking only close family or extended family too?)
now what do you do?
just take one member of your family, (who ever you're closest to, mum, dad, brother sister, son daughter) that one person will be killed unless you undertake a series of actions that will lead to WW3, and billions of deaths around the world.
the parameters here are 1 person from your family v.s 2 billion deaths, (1/3rd of the worlds population), but they are all strangers to you...
As has been said a couple of times now. truthfully you'd only be absolutely sure that you'd know what you were doing at the time.