Personally, I believe that the ends justify the means if the consequences are overall positive for the person doing the killing.
Kill one idiot to save a 100 respectable people? Fine by me.
Kill one stranger to save a person you care about? Fine by me also.
Say you killed my brother to save your brother. From your standpoint you've done something good. You've saved your brother. From my standpoint you've clearly done something wrong. Then it's up to me as to wether I should take some sort of revenge or let it pass.
I always look at it as the benefit to all people, while attempting to disclude myself. So if the benefit to one or more persons in society outweighs the costs to one or more persons in society, myself discluded, it is worth it.
So take something simple, like mowing my neighbor's lawn. The cost to me is time, and a little gas. There isn't really an immediate cost to anyone else (though indirect costs could be calculated, such as the opportunity cost of losing that time while I could be, say, mowing my own lawn, and now my wife has to mow our lawn instead because I don't have the time to do it. Or damage to the environment could be considered, if you believe in global warming, etc). The benefit is the neighbor has more time, and is happy that his neighbor was kind enough to do this for him.
So, I take these things into consideration, and put them on a scale.
Neighbor's time & happiness <> Time & money I could have serviced other people with
I wouldn't include my time or money as a cost directly, because I am to think of others before myself (and I absolutely do not do that enough).
Killing your brother to save my brother would equate in roughly the same global benefit as letting your brother live by letting my brother die, so I could not justify killing your brother. Applying my equation above is a little more difficult when taking into account human lives... actually, a lot more difficult. Makes good table talk discussion though.
"If you could kill one person to save two others, would you do it?"
On one hand, saving two lives is better than saving only one, but on the other hand, who says I am even in an adequate position or have all the necessary information to make such a decision?
Ok, I feel like I am rambling now, so I'm going to stop...
EDIT: BTW, I couldn't agree with you more about these things happening (lack of ethics) because of a lack of purpose in people's lives today. It's very true.