Not a very hard concept is it?
Yet even here a lot of people have trouble with it.
Life is a gift. Deliberate murder of the innocent is wrong.
Why is there even an argument here?
Original sin, Ted. And despair. Many people don't even think of their own lives as a gift - although they certainly consider them more valuable than other people's lives, especially if the other person is "in their way" in some fashion. Why did Cain kill Abel? Because he damaged Cain's self-esteem.
Why do people kill unborn children, or shoot at drivers who turned in front of them without signalling? Solipsism - "nothing really exists except me."