Just maths.

Of course, this has no moral or political bearing, I am no advocate of murder. Just a little unintuitive fact.

It plays on the ambiguity of the word "cause". Is the cause the reason why something was going to happen, or the reason why it happened precisely when it did? Say you set a stone on another at such a fragile balance that it could fall at any moment for any reason. Five minutes later, the wind blows and the stone falls. Was the cause of the fall you setting the stone where you did, or was it the wind? That's debatable. But now, assume you put the stone on a big snowball instead. As it gets warmer, whatever you do, the stone will end up on the floor. Say a squirrel jumps on the snowball and the stone falls before the snow has melted. If you see the stone at this time, and are asked why the stone is on the floor, you will certainly blame the squirrel. But if you only see the stone being set, the leave and come back in the summer. If asked why the stone is there, certainly you will think that the cause was that it was set on a snowball. But then, does it matter whether or not you know about the squirrel? If there had been a squirrel and the stone hadn't been set on the snowball, the stone wouldn't be one the floor there. But if the stone had been set on the snowball and there hadn't been a squirrel, it would now still be on the floor at roughly the same place. Therefore, the squirrel isn't the determining factor. However, whether you ask the question before of after the snow has melted changes your answer.

We think as murder (the squirrel) as the cause of death because we ask the question before the snow has melted (before a person's natural lifetime has elapsed). But if you consider it on the long term, the answer changes. The murder is no longer the cause of death.