Decision to kill–how does that mind work?

I’m a big fan of the Olympics and like many in the world, I was swept up last summer in the Oscar Pistorius story.  Those great stories about people who get to the Olympics against all odds are the biggest reason I watch.   So I’m really saddened to see the trouble he’s in.  And it’s reminding me of a question I’ve been asking myself for a while.

I don’t really get the decision to kill in any case, but I can sort of get how a mind might be working when a couple is married and there’s a lot of money involved.  Sort of.  But if a couple isn’t married so there are no legal hurdles to jump and no money to change hands and one of them has come to hate the other, what kind of mind looks at the options and decides murdering the other and facing a life sentence would be better than breaking up?

Since I believe that we’re all one and that whatever we see out there reflects something in all of us I really try to stand in the shoes of people in these situations and this is one I just don’t get.  I accept that if I’m seeing it it’s part of me but understanding how a mind gets there…  I just don’t get it.