Well in about 5 billion years or so the sun will run out of fuel. Amongst the various unpleasant phenomena that happen to a dying star of this kind, it will expand to somewhere out around Jupiters orbit (from memory). That ought to do the trick quite nicely.
If, on the other hand, you mean how will human life on Earth be wiped out, it will always come down to human stupidity - whether that's fucking around with things we're better off leaving alone, someone pressing the red button, or simply investing all our money in killing each other then having nothing practical to use when a big old asteroid comes lumbering along, it will always come down to our collective stupidity.