What would you do?

Ok so just watched life on Deathrow with Trevor McDonald, there were two stories that stuck with me..
The man who killed a mother and her 4 ( yes 4) year old daughter for NO reason! He said he regrets what he does and he knows he deserves the death penal and he knows how horrific his crimes were..
This is one of the many reasons I watch criminal minds, to see if there is ANY psychological or scientific explanation behind someone's thinking when they do something like this. He explained he was on Meth and he had intended to rape the mother but when he saw the child he couldn't bring himself to do it.. I mean so you can't rape the mum but you can kill her and the 4year old..what the fuck is wrong with you you sick fuck.. He said he felt he had gone too far and he thought if he killed them he would not go back to prison and get away with it.
How warped can you mind be?! He said he was abused and didn't have a good childhood (however I was quite impressed that he didn't use it as an excuse), but seriously what could you be thinking to kill two innocent people who have let you into their house because they think you are lost! He was very remorseful and I understand that he was on drugs but there must be something inside you that makes you capable of that, how can you let yourself get that bad.. To be soo high on drugs that you can do that!
I think there must be something inside you that after your childhood and tragic events in your life you turn to that, some people do the complete opposite ..
Why is it that some people use it to better themselves and make others proud and others use it to excuse terrible things that they do!?!?

Now on the other hand ( and I know a lot of people will be like wtf your contradicting yourself but hey) this other dude although he was paid (not much) like 700 quid to kill two people he said the reason he did it was because some guy had taken pictures of his family and said its AS easy to kill them as it is to get these photos, he explained that he thought the guy was a big shot and feared for his family's lives.
So in his mind his choice was them or the people he loved.. I can KIND of understand this ( kind of just so none of you go off on one thinking I'm gonna go on a killing spree) ..
He didn't ow these people which I would think makes its easier you COULD rationalise this "they might not have family" blah blah compared to your own family and the ones you love.
I know that regardless of how long I would spend in prison if I thought my family were going to be killed I would make that choice in a heartbeat..
Just imagine (and I know you can never know until it happens) someone had a gun to your mum/dad/sibling or child's head.. And said you kill some randomer or they die..
So what would you do?