I'm not sure it's simply black and white; I think there are a lot of grey areas in regards to that. It's highly likely I'm in the minority (or possibly even alone) with my thoughts on this but I believe every person is capable of terrible things, even murder; it's just a matter of vast differences in personal limits being reached, individual levels or abilities of restraint, and personal beliefs in regards to when something is permissable in their own mind regardless of the law.
I realize some will think this is like comparing apples to oranges, but the core of it is really the same: taking someone else's life or being someone who can kill or someone who cannot;
If I were to find myself under attack on the front lines of a war and it's me or them...you bet your



I'm going to shoot until I no longer have ammo.
Find myself the victim of a home invasion...they chose the wrong house to invade.
Find myself the victim of an assault/abduction/attempted murder....I've got too much to live for to go down without a massive fight.
So for myself, personally, if I am in a me or them/my life or theirs type of situation, I would definitely, without question be capable of killing someone. That is my (known) personal limit or times when I can knowingly justify it.
Each person is different. Some never think it is okay and would never harm another person. Others have much lower limits (road rage, gang retaliation, money, jealousy etc).
I suppose we all don't know our limits until we are face to face with them. Until that point, it's easy for the rest of us to point fingers or judge because "we would never...."
ETA: The Mathews obviously had a personal limit they had reached, if even in a fleeting moment. They don't get a pass from me either way; even if I too can admit to having my own personal limits.