system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can an asteroid of 6 miles in diameter cause such a global catastrophe on an earth nearly 8,000 miles in diameter?
gpt
By hitting at 30kps.  

The crater that it created was 150km in diameter.  That's a lot of material to remove.

But even before that you're talking about the impact itself releasing a lot of energy that would have created a fireball.  This would have set things on fire for quite a distance from it.

Now, back to the material from the crater.

A lot of it would have been blasted with such force that it would have gone into into space and then much of it would have re-entered the atmosphere.  This would have then caused the atmosphere to heat up like an oven over a lot of the planet.  If you couldn't seek refuge in a hole in the ground or deep water you cooked to death on much of the planet the very day the asteroid hit.

Now, the impact itself sent shockwaves through the Earth.  Very powerful ones.  There were Earthquakes, massive ones, all over the planet.  This then most likely also caused volcanoes to be set off or new ones to form.  A lot of toxic gases were most likely released into the atmosphere.  This would have poisoned a lot of life.

The material that wasn't thrown up with enough force to go into space floated around in the atmosphere for years.  This greatly reduced the amount of sunlight that reached the ground.  Most plants would have died.  What ate the plants then mostly died.  What ate the things that ate the plants then mostly died.

Most of life on Earth died because of all of this.