Well there is a lot more to "Ending Life as We know It" than just the size of an asteroid.
What is the asteroid made of? Is it hard enough to survive entering Earth atmosphere?
What is the trajectory of the asteroid? Will Earth get a glancing blow or a hard hit.
Where will the asteroid hit? Land, water, Siberia or the middle of the Mediterranean sea.
The last great extinction event caused by an asteroid was 65 million years ago and the asteroid was estimated to be 7 miles in diameter. More then 20 times the mass of this one, assuming both are a sphere. Bad assumption I know but it serves to illustrate just how much larger the 65 million ya was.
Yes it will be a bad day for living things on planet earth, but end life as we know it? Maybe, but then again maybe not.